PRACTICA 


ASTRONOMER 


HAROLD 


LIBRARY 

OF   THE 

UNIVERSITY  OF  CALIFORNIA. 


doss    157 

r 


PRACTICAL  TALKS    BY 
AN   ASTRONOMER 


The   Moon.      First   Quarter. 

Photographed   by   Loewy  and   Fuiseux,    February    13,    1894. 


PRACTICAL  TALKS   BY 
AN   ASTRONOMER 


BY 


HAROLD  JACOBY 

ADJUNCT   PROFESSOR   OF  ASTRONOMY   IN 
COLUMBIA   UNIVERSITY 


ILLUSTRATED 


OF  THE 

UNIVERSITY 

OF 


NEW   YORK 
CHARLES    SCRIBNER'S    SONS 

1902 


COPYRIGHT,  1902,  BY 
CHARLES  SCRIBNER'S  SONS 


Published,  April,  1902 


TROW   DIRECTORY 
,  AND   BOOKBINDING  COMPANY 
NEW   YORK 


PREFACE 

THE  present  volume  has  not  been  designed  as 
a  systematic  treatise  on  astronomy.  There  are 
many  excellent  books  of  that  kind,  suitable  for 
serious  students  as  well  as  the  general  reader ; 
but  they  are  necessarily  somewhat  dry  and  un- 
attractive, because  they  must  aim  at  complete- 
ness. Completeness  means  detail,  and  detail 
means  dryness. 

But  the  science  of  astronomy  contains  subjects 
that  admit  of  detached  treatment ;  and  as  many 
of  these  are  precisely  the  ones  of  greatest  general 
interest,  it  has  seemed  well  to  select  several,  and 
describe  them  in  language  free  from  technicalities. 
It  is  hoped  that  the  book  will  thus  prove  useful 
to  persons  who  do  not  wish  to  give  the  time 
required  for  a  study  of  astronomy  as  a  whole,  but 
who  may  take  pleasure  in  devoting  a  half-hour 

V 

101613 


PREFACE 

now  and  then  to  a  detached  essay  on  some  spe- 
cial topic. 

Preparation  of  the  book  in  this  form  has  made 
it  suitable  for  prior  publication  in  periodicals ; 
and  the  several  essays  have  in  fact  all  been 
printed  before.  But  the  intention  of  collecting 
them  into  a  book  was  kept  in  mind  from  the 
first;  and  while  no  attempt  has  been  made  at 
consecutiveness,  it  is  hoped  that  nothing  of 
merely  ephemeral  value  has  been  included. 


VI 


CONTENTS 

PAGE 

NAVIGATION  AT  SEA i 

THE  PLEIADES 10 

THE  POLE-STAR 18 

NEBULJE 271*** 

TEMPORARY  STARS 37 

GALILEO 47 

THE  PLANET  OF   1898 58 

How  TO  MAKE  A  SUN-DIAL 69 

PHOTOGRAPHY  IN  ASTRONOMY 81 

TIME  STANDARDS  OF  THE  WORLD 1 1 1 

MOTIONS  OF  THE  EARTH'S  POLE 131 

SATURN'S  RINGS  ...     * 140 

THE  HELIOMETER 152 

OCCULTATIONS         .       .       . l6l 

MOUNTING  GREAT  TELESCOPES 170 

THE  ASTRONOMER'S  POLE 184 

THE  MOON  HOAX 199 

THE  SUN'S  DESTINATION 210 


VI 1 


ILLUSTRATIONS 

THE  MOON.      FIRST  QUARTER Frontispiece 

Photographed  by  Loeivy  and  Puiseux,  February  /?,  1894. 

FACING 
PAGE 

SPIRAL  NEBULA  IN  CONSTELLATION  LEO 26 

Photographed  by  Keeler,  February  24,  iqoo, 

NEBULA  IN   ANDROMEDA 28 

Photographed  by  Barnard,  November  ii,  t8f)2. 

THE   " DUMB-BELL"   NEBULA 34 

Photographed  by  Keeler,  July  31,  iSQQ* 

STAR-FIELD  IN   CONSTELLATION  MONOCEROS      ....      84 

Photographed  by  Barnard,  February  /,  /<?<?./. 

SOLAR  CORONA.      TOTAL  ECLIPSE 1 08 

Photographed  by  Campbell,  January  23,  i8qS ;  Jeurt  India. 

FORTY-INCH  TELESCOPE,  YERKES  OBSERVATORY     .     .     .    1 70 
YERKES  OBSERVATORY,   UNIVERSITY  OF  CHICAGO  .     .      .176 


ix 


PRACTICAL    TALKS 
BY    AN    ASTRONOMER 

v  NAVIGATION    AT    SEA 

A  SHORT  time  ago  the  writer  had  occasion  to 
rummage  among  the  archives  of  the  Royal  As- 
tronomical Society  in  London,  to  consult,  if  pos- 
sible, the  original  manuscripts  left  by  one  Stephen 
Groombridge,  an  English  astronomer  of  the  good 
old  days,  who  died  in  1832.  It  was  known  that 
they  had  been  filed  away  about  a  generation  ago, 
by  the  late  Sir  George  Airy,  who  was  Astronomer 
Royal  of  England  between  the  years  1835  and 
1 88 1.  After  a  long  search,  a  large  and  dusty 
box  was  found  and  opened.  It  was  filled  with 
documents,  of  which  the  topmost  was  in  Sir 
George's  own  handwriting,  and  began  substan- 
tially as  follows : 

"  List  of  articles  within  this  box. 
"No.  i,  This  list, 

"  No.  2,  etc.,  etc." 

s 


NAVIGATION   AT   SEA 

Astronomical  precision  can  no  further  go :  he 
had  listed  even  the  list  itself.  Truly,  Airy  was 
rightly  styled  "  prince  of  precisians."  A  worthy 
Astronomer  Royal  was  he,  to  act  under  the 
royal  warrant  of  Charles  II.,  who  established  the 
office  in  1675.  Down  to  this  present  day  that 
warrant  still  makes  it  the  duty  of  His  Majesty's 
Astronomer  "  to  apply  himself  with  the  most 
exact  care  and  diligence  to  the  rectifying  of  the 
tables  of  the  motions  of  the  heavens  and  the 
places  of  the  fixed  stars,  in  order  to  find  out  the 
so  much  desired  longitude  at  sea,  for  the  per- 
fecting the  art  of  navigation." 

The  "  so  much  desired  longitude  at  sea "  is, 
indeed,  a  vastly  important  thing  to  a  maritime 
nation  like  England.  And  only  in  comparatively 
recent  years  has  it  become  possible  and  easy  for 
vessels  to  be  navigated  with  safety  and  conven- 
ience upon  long  voyages.  The  writer  was  well 
acquainted  with  an  old  sea-captain  of  New  York, 
who  had  commanded  one  of  the  earliest  transatlan- 
tic steamers,  and  who  died  only  a  few  years  ago. 
He  had  a  goodly  store  of  ocean  yarn,  fit  and 
ready  for  the  spinning,  if  he  could  but  find  some- 


NAVIGATION   AT   SEA 

one  who,  like  himself,  had  known  and  loved  the 
ocean.  In  his  early  sea-going  days,  only  the 
wealthiest  of  captains  owned  chronometers.  This 
instrument  is  now  considered  indispensable  in 
navigation,  but  at  that  time  it  was  a  new  in- 
vention, very  rare  and  costly.  Upon  a  certain 
voyage  from  England  to  Rio  Janeiro,  in  South 
America,  the  old  captain  could  remember  the 
following  odd  method  of  navigation  :  The  ship 
was  steered  by  compass  to  the  southward  and 
westward,  more  or  less,  until  the  skipper's 
antique  quadrant  showed  that  they  had  about 
reached  the  latitude  of  Rio.  Then  they  swung 
her  on  a  course  due  west  by  compass,  and  away 
she  went  for  Rio,  relying  on  the  lookout  man  for- 
ward to  keep  the  ship  from  running  ashore.  For 
after  a  certain  lapse  of  time,  being  ignorant  of  the 
longitude,  they  could  not  know  whether  they 
would  "  raise  "  the  land  within  an  hour  or  in  six 
weeks.  We  are  glad  of  an  opportunity  to  put 
this  story  on  record,  for  the  time  is  not  far  distant 
when  there  will  be  no  man  left  among  the  living 
who  can  remember  how  ships  were  taken  across 
the  seas  in  the  good  old  days  before  chronometers. 

3 


NAVIGATION   AT   SEA 

Anyone  who  has  ever  been  a  passenger  on  a 
great  transatlantic  liner  of  to-day  knows  what  an 
important,  imposing  personage  is  the  brass-bound 
skipper.  A  very  different  creature  is  he  on  the 
deck  of  his  ship  from  the  modest  seafaring  man  we 
meet  on  land,  clad  for  the  time  being  in  his  shore- 
going  togs.  But  the  captain's  dignity  is  not  all 
brass  buttons  and  gold  braid.  He  has  behind 
him  the  powerful  support  of  a  deep,  delightful 
mystery.  He  it  is  who  "  takes  the  sun "  at 
noon,  and  finds  out  the  ship's  path  at  sea.  And 
in  truth,  regarded  merely  as  a  scientific  experi- 
ment, the  guiding  of  a  vessel  across  the  unmarked 
trackless  ocean  has  few  equals  within  the  whole 
range  of  human  knowledge.  It  is  our  purpose 
here  to  explain  quite  briefly  the  manner  in  which 
this  seeming  impossibility  is  accomplished.  We 
shall  not  be  able  to  go  sufficiently  into  details  to 
enable  him  who  reads  to  run  and  navigate  a  mag- 
nificent steamer.  But  we  hope  to  diminish  some- 
what that  small  part  of  the  captain's  vast  dignity 
which  depends  upon  his  mysterious  operations 
with  the  sextant. 

To  begin,  then,  with  the  sextant  itself.     It  is 

4 


NAVIGATION  AT   SEA 

nothing  but  an  instrument  with  which  we  can 
measure  how  high  up  the  sun  is  in  the  sky.  Now, 
everyone  knows  that  the  sun  slowly  climbs  the 
sky  in  the  morning,  reaches  its  greatest  height  at 
noon,  and  then  slowly  sinks  again  in  the  after- 
noon. The  captain  simply  begins  to  watch  the 
sun  through  the  sextant  shortly  before  noon, 
and  keeps  at  it  until  he  discovers  that  the  sun  is 
just  beginning  to  descend.  That  is  the  instant  of 
noon  on  the  ship.  The  captain  quickly  glances 
at  the  chronometer,  or  calls  out  "  noon  "  to  an 
officer  who  is  near  that  instrument.  And  so  the 
error  of  the  chronometer  becomes  known  then 
and  there  without  any  further  astronomical  cal- 
culations whatever.  Navigators  can  also  find  the 
chronometer  error  by  sextant  observations  when 
the  sun  is  a  long  way  from  noon.  The  methods 
of  doing  this  are  somewhat  less  simple  than  for 
the  noon  observation ;  they  belong  to  the  details 
of  navigation,  into  which  we  cannot  enter  here. 

Incidentally,  the  captain  also  notes  with  the 
sextant  how  high  the  sun  was  in  the  sky  at  the 
noon  observation.  He  has  in  his  mysterious 
"  chart-room  "  some  printed  astronomical  tables, 

5 


NAVIGATION   AT   SEA 

which  tell  him  in  what  terrestrial  latitude  the  sun 
will  have  precisely  that  height  on  that  particular 
day  of  the  year.  Thus  the  terrestrial  latitude  be- 
comes known  easily  enough,  and  if  only  the  cap- 
tain could  get  his  longitude  too,  he  would  know 
just  where  his  ship  was  that  day  at  noon. 

We  have  seen  that  the  sextant  observations 
furnish  the  error  of  the  chronometer  according  to 
ship's  time.  In  other  words,  the  captain  is  in 
possession  of  the  correct  local  time  in  the  place 
where  the  ship  actually  is.  Now,  if  he  also  had 
the  correct  time  at  that  moment  of  some  well- 
known  place  on  shore,  he  would  know  the  differ- 
ence in  time  between  that  place  on  shore  and  the 
ship.  But  every  traveller  by  land  or  sea  is  aware 
that  there  are  always  differences  of  time  between 
different  places  on  the  earth.  If  a  watch  be 
right  on  leaving  New  York,  for  instance,  it  will 
be  much  too  fast  on  arriving  at  Chicago  or  San 
Francisco ;  the  farther  you  go  the  larger  becomes 
the  error  of  your  watch.  In  fact,  if  you  could 
find  out  how  much  your  watch  had  gone  into 
error,  you  would  in  a  sense  know  how  far  east  or 
west  you  had  travelled. 

6 


NAVIGATION   AT   SEA 

Now  the  captain's  chronometer  is  set  to  cor- 
rect "  Greenwich  time  "  on  shore  before  the  ship 
leaves  port.  His  observations  having  then  told 
him  how  much  this  is  wrong  on  that  particular 
day,  and  in  that  particular  spot  where  the  ship  is, 
he  knows  at  once  just  how  far  he  has  travelled 
east  or  west  from  Greenwich.  In  other  words, 
he  knows  his  "  longitude  from  Greenwich,"  for 
longitude  is  nothing  more  than  distance  from 
Greenwich  in  an  east-and-west  direction,  just  as 
latitude  is  only  distance  from  the  equator  meas- 
ured in  a  north-and-south  direction.  Greenwich 
observatory  is  usually  selected  as  the  beginning 
of  things  for  measuring  longitudes,  because  it  is 
almost  the  oldest  of  existing  astronomical  estab- 
lishments, and  belongs  to  the  most  prominent 
maritime  nation,  England. 

One  of  the  most  interesting  bits  of  astronomi- 
cal history  was  enacted  in  connection  with  this 
matter  of  longitude.  From  what  has  been  said, 
it  is  clear  that  the  ship's  longitude  will  be  ob- 
tained correctly  only  if  the  chronometer  has  kept 
exact  time  since  the  departure  of  the  ship  from 
port.  Even  a  very  small  error  of  the  chronom- 

7 


NAVIGATION   AT   SEA 

eter  will  throw  out  th-e  longitude  a  good  many 
miles,  and  we  can  understand  readily  that  it  must 
be  difficult  in  the  extreme  to  construct  a  mechan- 
ical contrivance  capable  of  keeping  exact  time 
when  subjected  to  the  rolling  and  pitching  of  a 
vessel  at  sea. 

It  was  as  recently  as  the  year  1736  that  the 
first  instrument  capable  of  keeping  anything  like 
accurate  time  at  sea  was  successfully  completed. 
It  was  the  work  of  an  English  watchmaker 
named  John  Harrison,  and  is  one  of  the  few 
great  improvements  in  matters  scientific  which 
the  world  owes  to  a  desire  for  winning  a  money 
prize.  It  appears  that  in  1714  a  committee  was 
appointed  by  the  House  of  Commons,  with  no 
less  a  person  than  Sir  Isaac  Newton  himself  as 
one  of  its  members,  to  consider  the  desirability 
of  offering  governmental  encouragement  for  the 
invention  of  some  means  of  finding  the  longitude 
at  sea.  Finally,  the  British  Government  offered 
a  reward  of  $50,000  for  an  instrument  which 
would  find  the  longitude  within  sixty  miles ; 
$75,000,  if  within  forty  miles,  and  $100,000,  if 
within  thirty  miles.  Harrison's  chronometer  was 


NAVIGATION   AT   SEA 

finished  in  1736,  but  he  did  not  receive  the  final 
payment  of  his  prize  until  1764. 

We  shall  not  enter  into  a  detailed  account  of 
the  vexatious  delays  and  official  procedures  to 
which  he  was  forced  to  submit  during  those 
twenty-eight  long  years.  It  is  a  matter  of  satis- 
faction that  Harrison  lived  to  receive  the  money 
which  he  had  earned.  He  had  the  genius  to 
plan  and  master  intricate  mechanical  details,  but 
perhaps  he  lacked  in  some  degree  the  ability  of 
tongue  and  pen  to  bring  them  home  to  others. 
This  may  be  the  reason  he  is  so  little  known, 
though  it  was  his  fortune  to  contribute  so  large 
and  essential  a  part  to  the  perfection  of  modern 
navigation.  Let  us  hope  this  brief  mention  may 
serve  to  recall  his  memory  from  oblivion  even  for 
a  fleeting  moment ;  that  we  may  not  have  written 
in  vain  of  that  longitude  to  which  his  life  was 
given. 


THE    PLEIADES 

FAMED  in  legend ;  sung  by  early  minstrels  of 
Persia  and  Hindustan ; 

'* — like  a  swarm  of  fire-flies  tangled  in  a  silver  braid  "  ; 

yonder  distant  misty  little  cloud  of  Pleiades  has 
always  won  and  held  the  imagination  of  men. 
But  it  was  not  only  for  the  inspiration  of  poets, 
for  quickening  fancy  into  song,  that  the  seven 
daughters  of  Atlas  were  fixed  upon  the  firma- 
ment. The  problems  presented  by  this  group  of 
stars  to  the  unobtrusive  scientific  investigator  are 
among  the  most  interesting  known  to  astronomy. 
Their  solution  is  still  very  incomplete,  but  what 
we  have  already  learned  may  be  counted  justly 
among  the  richest  spoils  brought  back  by  sci- 
ence from  the  stored  treasure-house  of  Nature's 
secrets. 

The  true  student  of  astronomy  is  animated  by 
no  mere  vulgar  curiosity  to  pry  into  things  hid- 
den. If  he  seeks  the  concealed  springs  that 


THE   PLEIADES 

move  the  complex  visible  mechanism  of  the 
heavens,  he  does  so  because  his  imagination  is 
roused  by  the  grandeur  of  what  he  sees ;  and 
deep  down  within  him  stirs  the  true  love  of  the 
artist  for  his  art.  For  it  is  indeed  a  fine  art,  that 
science  of  astronomy. 

It  can  have  been  no  mere  chance  that  has 
massed  the  Pleiades  from  among  their  fellow 
stars.  Men  of  ordinary  eyesight  see  but  a  half- 
dozen  distinct  objects  in  the  cluster ;  those  of 
acuter  vision  can  count  fourteen ;  but  it  is  not 
until  we  apply  the  space-penetrating  power  of 
the  telescope  that  we  realize  the  extraordinary 
scale  upon  which  the  system  of  the  Pleiades  is 
constructed.  With  the  Paris  instrument  Wolf  in 
1876  catalogued  625  stars  in  the  group;  and  the 
searching  photographic  survey  of  Henry  in  1887 
revealed  no  less  than  2,326  distinct  stars  within 
and  near  the  filmy  gauze  of  nebulous  matter  al- 
ways so  conspicuous  a  feature  of  the  Pleiades. 

The  means  at  our  disposal  for  the  study  of 
stellar  distances  are  but  feeble.  Only  in  the  case 
of  a  very  small  number  of  stars  have  we  been 

able  to  obtain  even  so  much  as  an  approximate 

ii 


THE   PLEIADES 

estimate  of  distance.  The  most  powerful  obser- 
vational machinery,  though  directed  by  the  tried 
skill  of  experience,  has  not  sufficed  to  sound  the 
profounder  depths  of  space.  The  Pleiad  stars 
are  among  those  for  which  no  measurement  of 
distance  has  yet  been  made,  so  that  we  do  not 
know  whether  they  are  all  equally  far  away  from 
us.  We  see  them  projected  on  the  dark  back- 
ground of  the  celestial  vault ;  but  we  cannot  tell 
from  actual  measurement  whether  they  are  all 
situated  near  the  same  point  in  space.  It  may  be 
that  some  are  immeasurably  closer  to  us  than  are 
the  great  mass  of  their  companions ;  possibly  we 
look  through  the  cluster  at  others  far  behind  it, 
clinging,  as  it  were,  to  the  very  fringe  of  the  visi- 
ble universe. 

Farther  on  we  shall  find  evidence  that  some- 
thing like  this  really  is  the  case.  But  under  no 
circumstances  is  it  reasonable  to  suppose  that  the 
whole  body  of  stars  can  be  strung  out  at  all  sorts 
of  distances  near  a  straight  line  pointing  in  the 
direction  of  the  visible  cluster.  Such  a  distribu- 
tion may  perhaps  remain  among  the  possibilities, 
so  long  as  we  cannot  measure  directly  the  actual 


12 


THE   PLEIADES 

distances  of  the  individual  stars.  But  science 
never  accepts  a  mere  possibility  against  which  we 
can  marshal  strong  circumstantial  evidence.  We 
may  conclude  on  general  principles  that  the 
gathering  of  these  many  objects  into  a  single 
close  assemblage  denotes  community  of  origin 
and  interests. 

The  Pleiades  then  really  belong  to  one  an- 
other. What  is  the  nature  of  their  mutual  tie  ? 
What  is  their  mystery,  and  can  we  solve  it  ? 
The  most  obvious  theory  is,  of  course,  suggested 
by  what  we  know  to  be  true  within  our  own 
solar  system.  We  owe  to  Newton  the  beautiful 
conception  of  gravitation,  that  unique  law  by 
means  of  which  astronomers  have  been  en- 
abled to  reduce  to  perfect  order  the  seeming 
tangle  of  planetary  evolutions.  The  law  really 
amounts,  in  effect,  to  this :  All  objects  suspended 
within  the  vacancy  of  space  attract  or  pull  one 
another.  How  they  can  do  this  without  a  visi- 
ble connecting  link  between  them  is  a  mystery 
which  may  always  remain  unsolved.  But  mys- 
tery as  it  is,  we  must  accept  it  as  an  ascertained 
fact.  It  is  this  pull  of  gravitation  that  holds  to- 

13 


THE   PLEIADES 

gather  the  sun  and  planets,  forcing  them  all  to 
follow  out  their  due  and  proper  paths,  and  so  to 
continue  throughout  an  unbroken  cycle  until  the 
great  survivor,  Time,  shall  be  no  more. 

This  same  gravitational  attraction  must  be  at 
work  among  the  Pleiades.  They,  too,  like  our- 
selves, must  have  bounds  and  orbits  set  and 
interwoven,  revolutions  and  gyrations  far  more 
complex  than  the  solar  system  knows.  The 
visual  discovery  of  such  motion  of  rotation 
among  the  Pleiades  may  be  called  one  of  the 
pressing  problems  of  astronomy  to-day.  We 
feel  sure  that  the  time  is  ripe,  and  that  the  dis- 
covery is  actually  being  made  at  the  present  mo- 
ment :  for  a  generation  of  men  is  not  too  great  a 
period  to  call  a  moment,  when  we  have  to  deal 
with  cosmic  time. 

It  is  indeed  the  lack  of  observations  extending 
through  sufficient  centuries  that  stays  our  hand 
from  grasping  the  coveted  result.  The  Pleiades 
are  so  far  from  us  that  we  cannot  be  sure  of 
changes  among  them.  Magnitudes  are  always 
relative.  It  matters  not  how  large  the  actual 
movements  may  be  ;  if  they  are  extremely  small 

14 


THE   PLEIADES 

in  comparison  with  our  distance,  they  must 
shrink  to  nothingness  in  our  eyes.  Trembling 
on  the  verge  of  invisibility,  elusive,  they  are  in 
that  borderland  where  science  as  yet  but  feels  her 
way,  though  certain  that  the  way  is  there. 

The  foundations  of  exact  modern  knowledge 
of  the  group  were  laid  by  Bessel  about  1840. 
With  the  modesty  characteristic  of  the  great,  he 
says  quite  simply  that  he  has  made  a  number  of 
measures  of  the  Pleiades,  thinking  that  the  time 
may  come  when  astronomers  will  be  able  to  find 
some  evidence  of  motion.  In  this  unassuming 
way  he  prefaces  what  is  still  the  classic  model  of 
precision  and  thoroughness  in  work  of  this  kind. 
Bessel  cleared  the  ground  for  a  study  of  inter- 
stellar motion  within  the  close  star-clusters  ;  and 
it  is  probable  that  only  by  such  study  may  we 
hope  to  demonstrate  the  universality  of  the  law 
of  gravitation  in  cosmic  space. 

Bessel's  acuteness  in  forecasting  the  direction 
of  coming  research  was  amply  verified  by  the 
work  of  Elkin  in  1885  at  Yale  College.  Pro- 
vided with  a  more  modern  instrument,  but  sim- 
ilar to  Bessel's,  Elkin  was  able  to  repeat  his 

15 


THE   PLEIADES 

observations  with  a  slight  increase  of  precision. 
Motions  in  the  interval  of  forty-five  years,  suffi- 
ciently great  to  hint  at  coming  possibilities,  were 
shown  conclusively  to  exist.  Six  stars  at  all 
events  have  been  fairly  excluded  from  the  group 
on  account  of  their  peculiar  motions  shown  by 
Elkin's  research.  It  is  possible  that  they  are 
merely  seen  in  the  background  through  the  in- 
terstices of  the  cluster  itself,  or  they  may  be  sus- 
pended between  us  and  the  Pleiades,  in  either 
case  having  no  real  connection  with  the  group. 
Finally,  these  observations  make  it  reasonably 
certain  that  many  of  the  remaining  mass  of  stars 
really  constitute  a  unit  aggregation  in  space. 
Astronomers  of  a  coming  generation  will  again 
repeat  the  Besselian  work.  At  present  we  have 
been  able  to  use  his  method  only  for  the  separa- 
tion from  the  true  Pleiades  of  chance  stars  that 
happen  to  lie  in  the  same  direction.  Let  us  hope 
that  man  shall  exist  long  enough  upon  this  earth 
to  see  the  clustered  stars  themselves  begin  and 
carry  out  such  gyrations  as  gravitation  imposes. 

These  will  doubtless   be   of  a  kind  not  even 
suggested  by  the  lesser  complexities  of  our  solar 

16 


THE   PLEIADES 

system.  For  the  most  wonderful  thing  of  all 
about  the  Pleiades  seems  to  point  to  an  intricacy 
of  structure  whose  details  may  be  destined  to 
shake  the  confidence  of  the  profoundest  mathe- 
matician. There  is  an  extraordinary  nebulous 
condensation  that  seems  to  pervade  the  entire 
space  occupied  by  the  stellar  constituents  of  the 
group.  The  stars  are  swimming  in  a  veritable 
sea  of  luminous  cloud.  There  are  filmy  tenuous 
places,  and  again  condensing  whirls  of  material 
doubtless  still  in  the  gaseous  or  plastic  stage. 
Most  noticeable  of  all  are  certain  almost  straight 
lines  of  nebula  that  connect  series  of  stars.  In 
one  case,  shown  upon  a  photograph  made  by 
Henry  at  Paris,  six  stars  are  strung  out  upon 
such  a  hazy  line.  We  might  give  play  to  fancy, 
and  see  in  this  the  result  of  some  vast  eruption 
of  gaseous  matter  that  has  already  begun  to 
solidify  here  and  there  into  stellar  nuclei.  But 
sound  science  gives  not  too  great  freedom  to 
mere  speculative  theories.  Her  duty  has  been 
found  in  quiet  research,  and  her  greatest  rewards 
have  flowed  from  imaginative  speculation,  only 
when  tempered  by  pure  reason. 

17 


THE    POLE-STAR 

ONE  of  the  most  brilliant  observations  of  the 
last  few  years  is  Campbell's  recent  discovery  of 
the  triple  character  of  this  star.  Centuries  and 
centuries  ago,  when  astronomy,  that  venerable 
ancient  among  the  sciences,  was  but  an  infant, 
the  pole-star  must  have  been  considered  the  very 
oldest  of  observed  heavenly  bodies.  In  the  be- 
ginning it  was  the  only  sure  guide  of  the  naviga- 
tor at  night,  just  as  to  this  day  it  is  the  founda- 
tion-stone for  all  observational  stellar  astronomy 
of  precision.  There  has  never  been  a  time  in 
the  history  of  astronomy  when  the  pole-star 
might  not  have  been  called  the  most  frequently 
measured  object  in  the  sky  of  night.  So  it  is 
indeed  strange  that  we  should  find  out  some- 
thing altogether  new  about  it  after  all  these  ages 
of  study. 

But  the  importance  of  the  discovery  rests 
upon  a  surer  foundation  than  this.  The  method 

18 


THE   POLE-STAR 

by  which  it  has  been  made  is  almost  a  new  one 
in  the  science.  A  generation  ago,  men  thought 
the  "  perfect  science,"  for  so  we  love  to  call 
astronomy,  could  advance  only  by  increasing  a 
little  the  exact  precision  of  observation.  The 
citadel  of  perfect  truth  might  be  more  closely  in- 
vested ;  the  forces  of  science  might  push  forward 
step  by  step  ;  the  machinery  of  research  might 
be  strengthened,  but  that  a  new  engine  of  inves- 
tigation would  be  discovered  capable  of  penetrat- 
ing where  no  telescope  can  ever  reach,  this,  in- 
deed, seemed  far  beyond  the  liveliest  hope  of 
science.  Even  the  discoverer  of  the  spectro- 
scope could  never  have  dreamed  of  its  possibil- 
ities, could  never  have  foreseen  its  successes,  its 
triumphs. 

The  very  name  of  this  instrument  suggests 
mystery  to  the  popular  mind.  It  is  set  down  at 
once  among  the  things  too  difficult,  too  intricate, 
too  abstruse  to  understand.  Yet  in  its  essentials 
there  is  nothing  about  the  spectroscope  that  can- 
not be  made  clear  in  a  few  words.  Even  the 
modern  "  undulatory  theory  "  of  light  itself  is 
terrible  only  in  the  length  of  its  name.  Any- 


THE   POLE-STAR 

one  who  has  seen  the  waves  of  ocean  roll,  roll, 
and  ever  again  roll  in  upon  the  shore,  can  form  a 
very  good  notion  of  how  light  moves.  'Tis  just 
such  a  series  of  rolling  waves ;  started  perhaps 
from  some  brilliant  constellation  far  out  upon  the 
confining  bounds  of  the  visible  universe,  or  per- 
haps coming  from  a  humble  light  upon  the  stu- 
dent's table ;  yet  it  is  never  anything  but  a 
succession  of  rolling  waves.  Only,  unlike  the 
waves  of  the  sea,  light  waves  are  all  excessively 
small.  We  should  call  one  whose  length  was  a 
twenty-thousandth  of  an  inch  a  big  one  ! 

Now  the  human  eye  possesses  the  property  of 
receiving  and  understanding  these  little  waves. 
The  process  is  an  unconscious  one.  Let  but  a 
set  of  these  tiny  waves  roll  up,  as  it  were,  out  of 
the  vast  ocean  of  space  and  impinge  upon  the  eye, 
and  all  the  phenomena  of  light  and  color  become 
what  we  call  "visible."  We  see  the  light. 

And  how  does  all  this  find  an  application  in 
astronomy  ?  Not  to  enter  too  much  into  technical 
details,  we  may  say  that  the  spectroscope  is  an  in- 
strument which  enables  us  to  measure  the  length 
of  these  light  waves,  though  their  length  is  so 


THE   POLE-STAR 

exceedingly  small.  The  day  has  indeed  gone  by 
when  that  which  poets  love  to  call  the  Book  of 
Nature  was  printed  in  type  that  could  be  read 
by  the  eye  unaided.  Telescope,  microscope,  and 
spectroscope  are  essential  now  to  him  who  would 
penetrate  any  of  Nature's  secrets.  But  measure- 
ments with  a  telescope,  like  eye  observations,  are 
limited  strictly  to  determining  the  directions  in 
which  we  see  the  heavenly  bodies.  Ever  since 
the  beginning  of  things,  when  old  Hipparchus  and 
Ulugh  Beg  made  the  first  rude  but  successful  at- 
tempts to  catalogue  the  stars,  the  eye  and  telescope 
have  been  able  to  measure  only  such  directions. 
We  aim  the  telescope  at  a  star,  and  record  the 
direction  in  which  it  was  pointed.  Distances  in 
astronomy  can  never  be  measured  directly.  All 
that  we  know  of  them  has  been  obtained  by  cal- 
culations based  upon  the  Newtonian  law  of  gravi- 
tation and  observations  of  directions. 

Now  the  spectroscope  seems  to  offer  a  sort  of 
exception  to  this  rule.  Suppose  we  can  measure 
the  wave-lengths  of  the  light  sent  us  from  a  star. 
Suppose  again  that  the  star  is  itself  moving  swiftly 
toward  us  through  space,  while  continually  set- 

21 


THE   POLE-STAR 

ting  in  motion  the  waves  of  light  that  are  ulti- 
mately to  reach  the  waiting  astronomer.  Evi- 
dently the  light  waves  will  be  crowded  together 
somewhat  on  account  of  the  star's  motion.  More 
waves  per  second  will  reach  us  than  would  be  re- 
ceived from  a  star  at  rest.  It  is  as  though  the 
light  waves  were  compressed  or  shortened  a  little. 
And  if  the  star  is  leaving  us,  instead  of  coming 
nearer,  opposite  effects  will  occur.  We  have  then 
but  to  compare  spectroscopically  starlight  with 
some  artificial  source  of  light  in  the  observatory  in 
order  to  find  out  whether  the  star  is  approaching 
us  or  receding  from  us.  And  by  a  simple  process 
of  calculation  this  stellar  motion  can  be  obtained 
in  miles  per  second.  Thus  we  can  now  actually 
measure  directly,  in  a  certain  sense,  linear  speed 
in  stellar  space,  though  we  are  still  without  the 
means  of  getting  directly  at  stellar  distances. 

But  the  most  wonderful  thing  of  all  about  these 
spectroscopic  measures  is  the  fact  that  it  makes 
no  difference  whatever  how  far  away  is  the  star 
under  observation.  What  we  learn  through  the 
spectroscope  comes  from  a  study  of  the  waves 
themselves,  and  it  is  of  no  consequence  how  far 

22 


THE   POLE-STAR 

they  have  travelled,  or  how  long  they  have  been 
a-coming.  For  it  must  not  be  supposed  that 
these  waves  consume  no  time  in  passing  from  a 
distant  star  to  our  own  solar  system.  It  is  true 
that  they  move  exceeding  fast;  certainly  180,000 
miles  per  second  may  be  called  rapid  motion. 
But  if  this  cosmic  velocity  of  light  is  tremendous, 
so  also  are  cosmic  distances  correspondingly  vast. 
Light  needs  to  move  quickly  coming  from  a  star, 
for  even  at  the  rate  of  motion  we  have  mentioned 
it  requires  many  years  to  reach  us  from  some  of 
the  more  distant  constellations.  It  has  been  well 
said  that  an  observer  on  some  far-away  star,  if 
endowed  with  the  power  to  see  at  any  distance, 
however  great,  might  at  this  moment  be  looking 
on  the  Crusaders  proceeding  from  Europe  against 
the  Saracen  at  Jerusalem.  For  it  is  quite  pos- 
sible that  not  until  now  has  the  light  which  would 
make  the  earth  visible  had  time  to  reach  him. 
Yet  distant  as  such  an  observer  might  be,  light 
from  the  star  on  which  he  stood  could  be  meas- 
ured in  the  spectroscope,  and  would  infallibly  tell 
us  whether  the  earth  and  star  are  approaching  in 
space  or  gradually  drawing  farther  asunder. 

23 


THE   POLE-STAR 

The  pole-star  is  not  one  of  the  more  distant 
stellar  systems.  We  do  not  know  how  far  it  is 
from  us  very  exactly,  but  certainly  not  less  than 
forty  or  fifty  years  are  necessary  for  its  light  to 
reach  us.  The  star  might  have  gone  out  of  ex- 
istence twenty  years  ago,  and  we  not  yet  know 
of  it,  for  we  would  still  be  receiving  the  light 
which  began  its  long  journey  to  us  about  1850  or 
1860.  But  no  matter  what  may  be  its  distance, 
Campbell  found  by  careful  observations,  made  in 
the  latter  part  of  1896,  that  the  pole-star  was  then 
approaching  the  earth  at  the  rate  of  about  twelve 
miles  per  second.  So  far  there  was  nothing  espe- 
cially remarkable.  But  in  August  and  September 
of  the  present  year  twenty-six  careful  determina- 
tions were  made,  and  these  showed  that  now  the 
rate  of  approach  varied  between  about  five  and 
nine  miles  per  second.  More  astonishing  still, 
there  was  a  uniform  period  in  the  changes  of 
velocity.  In  about  four  days  the  rate  of  motion 
changed  from  about  five  to  nine  miles  and  back 
again.  And  this  variation  kept  on  with  great 
regularity.  Every  successive  period  of  four  days 
saw  a  complete  cycle  of  velocity  change  forward 

24 


THE   POLE-STAR 

and  back  between  the  same  limits.  There  can  be 
but  one  reasonable  explanation.  This  star  must 
be  a  double,  or  "  binary  "  star.  The  two  com- 
ponents, under  the  influence  of  powerful  mutual 
gravitational  attraction,  must  be  revolving  in  a 
mighty  orbit.  Yet  this  vast  orbit,  as  a  whole,  with 
the  two  great  stars  in  it,  must  be  approaching  our 
part  of  the  universe  all  the  time.  For  the  spectro- 
scope shows  the  velocity  of  approach  to  increase 
and  diminish,  indeed,  but  it  is  always  present. 
Here,  then,  is  this  great  stellar  system,  having  a 
four-day  revolution  of  its  own,  and  yet  swinging 
rapidly  through  space  in  our  direction.  Nor  is 
this  all.  One  of  the  component  stars  must  be 
nearly  or  quite  dark ;  else  its  presence  would  in- 
fallibly be  detected  by  our  instruments. 

And  now  we  come  to  the  most  astonishing 
thing  of  all.  How  comes  it  that  the  average 
rate  of  approach  of  the  "  four-day  system,"  as  a 
whole,  changed  between  1896  and  1899?  In 
1896  only  this  velocity  of  the  whole  system  was 
determined,  the  four-day  period  remaining  undis- 
covered until  the  more  numerous  observations  of 
1899.  But  even  without  considering  the  four- 

25 


THE   POLE-STAR 

day  period,  the  changing  velocity  of  the  entire 
system  offers  one  of  those  problems  that  exact 
science  can  treat  only  by  the  help  of  the  imagina- 
tion. There  must  be  some  other  great  centre  of 
attraction,  some  cosmic  giant,  holding  the  visible 
double  pole-star  under  its  control.  Thus,  that 
which  we  see,  and  call  the  pole-star,  is  in  reality 
threading  its  path  about  the  third  and  greatest 
member  of  the  system,  itself  situated  in  space,  we 
know  not  where. 


26 


Spiral  Nebula  in  Constellation  Leo. 

Photographed  by   Keeler,    February  24,    1900. 

Exposure,  three  hours,  fifty  minutes. 


/'  OF  THE 

((    UNIVERSITY   ] 

OF 


NEBULA 

SCATTERED  about  here  and  there  among  the 
stars  are  certain  patches  of  faint  luminosity  called 
by  astronomers  Nebulae.  These  "  little  clouds" 
of  filmy  light  are  among  the  most  fascinating  of 
all  the  kaleidoscopic  phenomena  of  the  heavens  ; 
for  it  needs  but  a  glance  at  one  of  them  to  give 
the  impression  that  here  before  us  is  the  stuff  of 
which  worlds  are  made.  All  our  knowledge  of 
Nature  leads  us  to  expect  in  her  finished  work 
the  result  of  a  series  of  gradual  processes  of  de- 
velopment. Highly  organized  phenomena  such 
as  those  existing  in  our  solar  system  did  not 
spring  into  perfection  in  an  instant.  Influential 
forces,  easy  to  imagine,  but  difficult  to  define, 
must  have  directed  the  slow,  sure  transformation 
of  elemental  matter  into  sun  and  planets,  things 
and  men.  Therefore  a  study  of  those  forces 
and  of  their  probable  action  upon  nebular 
material  has  always  exerted  a  strong  attraction 

27 


NEBULAE 

upon  the  acutest  thinkers  among  men  of  exact 
science. 

Our  knowledge  of  the  nebulae  is  of  two  kinds — 
that  which  has  been  ascertained  from  observation 
as  to  their  appearance,  size,  distribution,  and  dis- 
tance ;  and  that  which  is  based  upon  hypotheses 
and  theoretical  reasoning  about  the  condensation 
of  stellar  systems  out  of  nebular  masses.  It  so 
happens  that  our  observational  material  has  re- 
ceived a  very  important  addition  quite  recently 
through  the  application  of  photography  to  the 
delineation  of  nebulae,  and  this  we  shall  describe 
farther  on. 

Two  nebulae  only  are  visible  to  the  unaided 
eye.  The  brighter  of  these  is  in  the  constellation 
Andromeda  ;  it  is  of  oval  or  elliptical  shape,  and 
has  a  distinct  central  condensation  or  nucleus. 
Upon  a  photograph  by  Roberts  it  appears  to 
have  several  concentric  rings  surrounding  the 
nebula  proper,  and  gives  the  general  impression 
of  a  flat  round  disk  foreshortened  into  an  oval 
shape  on  account  of  the  observer's  position  not 
being  square  to  the  surface  of  the  disk.  Very 
recent  photographs  of  this  nebula,  made  with  the 

28 


Nebula  in  Andromeda. 

Lower  object  in  the  photograph  is  a  Comet. 
Photographed  by  Barnard,   November  21,    1892. 


NEBULAE 

three-foot  reflecting  telescope  of  the  Lick  Ob- 
servatory, bring  out  the  fact  that  it  is  really  spiral 
in  form,  and  that  the  outlying  nebulous  rings  are 
only  parts  of  the  spires  in  a  great  cosmic  whorl. 

This  Andromeda  nebula  is  the  one  in  which 
the  temporary  star  of  1885  appeared.  It  blazed 
up  quite  suddenly  near  the  apparent  centre  of  the 
nebula,  and  continued  in  view  for  six  months, 
fading  finally  beyond  the  reach  of  our  most 
powerful  telescopes.  There  can  be  little  doubt 
that  the  star  was  actually  in  the  nebula,  and  not 
merely  seen  through  it,  though  in  reality  situated 
in  the  extreme  outlying  part  of  space  at  a  distance 
immeasurably  greater  than  that  separating  us  from 
the  nebula  itself.  Such  an  accidental  superpo- 
sition of  nebula  and  star  might  even  be  due  to 
sudden  incandescence  of  a  new  star  between  us 
and  the  nebula.  In  such  a  case  we  should  see 
the  star  projected  upon  the  surface  of  the  nebula, 
so  that  the  superposition  would  be  identical  with 
that  actually  observed.  Therefore,  while  it  is, 
indeed,  possible  that  the  star  may  have  been  either 
far  behind  the  nebula  or  in  front  of  it,  we  must 
accept  as  more  probable  the  supposition  that 

29 


NEBULAE 

there  was  a  real  connection  between  the  two.  In 
that  case  there  is  little  doubt  that  we  have  actu- 
ally observed  one  of  those  cataclysms  that  mark 
successive  steps  of  cosmic  evolution.  We  have 
no  thoroughly  satisfactory  theory  to  account  for 
such  an  explosive  catastrophe  within  the  body  of 
the  nebula  itself. 

The  other  naked-eye  nebula  is  in  the  constella- 
tion Orion.  In  the  telescope  it  is  a  more  strik- 
ing object,  perhaps,  than  the  Andromeda  nebula ; 
for  it  has  no  well-defined  geometrical  form,  but 
consists  of  an  immense  odd-shaped  mass  of  light 
enclosing  and  surrounding  a  number  of  stars.  It 
is  unquestionably  of  a  very  complicated  structure, 
and  is,  therefore,  less  easily  studied  and  explained 
than  the  nebulae  of  simpler  form.  There  is  no 
doubt  that  the  Orion  nebula  is  composed  of  lumi- 
nous gas,  and  is  not  merely  a  cluster  of  small 
stars  too  numerous  and  too  near  together  to  be 
separated  from  each  other,  even  in  our  most 
powerful  telescopes.  It  was,  indeed,  supposed, 
until  about  forty  years  ago,  that  all  the  nebulae 
are  simply  irresolvable  star-clusters ;  but  we  now 
have  indisputable  evidence,  derived  from  the 

30 


NEBULAE 

spectroscope,  that  many  nebulae  are  composed  of 
true  gases,  similar  to  those  with  which  we  experi- 
ment in  chemical  laboratories.  This  spectro- 
scopic  proof  of  the  gaseous  character  of  nebulae  is 
one  of  the  most  important  discoveries  contrib- 
uted by  that  instrument  to  our  small  stock  of 
facts  concerning  the  structure  of  the  sidereal  uni- 
verse. 

Coming  now  to  the  smaller  nebulae,  we  find  a 
great  diversity  of  form  and  appearance.  Some 
are  ring-shaped,  perhaps  having  a  less  brilliant 
nebulosity  within  the  ring.  Many  show  a  central 
condensation  of  disk-like  appearance  (planetary 
nebulae),  or  have  simply  a  star  at  the  centre 
(nebulous  stars).  Altogether  about  ten  thousand 
such  objects  have  been  catalogued  by  successive 
generations  of  astronomers  since  the  invention  of 
the  telescope,  and  most  of  these  have  been  re- 
ported as  oval  in  form.  Now  we  have  already 
referred  to  the  important  addition  to  our  knowl- 
edge of  the  nebulae  obtained  by  recent  photo- 
graphic observations  ;  and  this  addition  consists  in 
the  discovery  that  most  of  these  oval  nebulae  are 
in  reality  spirals.  Indeed,  it  appears  that  the  spiral 

31 


NEBULAE 

type  is  the  normal  type,  and  that  nebulae  of  irreg- 
ular or  other  forms  are  exceptions  to  the  gen- 
eral rule.  Even  the  great  Andromeda  nebula,  as 
we  have  seen,  is  now  recognized  as  a  spiral. 

The  instrument  with  which  its  convolute 
structure  was  discovered  is  a  three-foot  reflecting 
telescope,  made  by  Common  of  England,  and  now 
mounted  at  the  Lick  Observatory,  in  California. 
The  late  Professor  Keeler  devoted  much  of  his 
time  to  photographing  nebulae  during  the  last  year 
or  two.  He  was  able  to  establish  the  important 
fact  just  mentioned,  that  most  nebulae  formerly 
thought  to  be  mere  ovals,  turn  out  to  be  spiral 
when  brought  under  the  more  searching  scrutiny 
of  the  photographic  plate  applied  at  the  focus  of 
a  telescope  of  great  size,  and  with  an  exposure  to 
the  feeble  nebular  light  extending  through  three 
or  four  consecutive  hours. 

Many  of  the  spirals  have  more  than  a  single 
volute.  It  is  as  though  one  were  to  attach  a 
number  of  very  flexible  rods  to  an  axle,  like 
spokes  of  a  wheel  without  a  rim  and  then  revolve 
the  axle  rapidly.  The  flexible  rods  would  bend 
under  the  rapid  rotation,  and  form  a  series  of 

32 


NEBULAE 

spiral  curves  not  unlike  many  of  these  nebulae. 
Indeed,  it  is  impossible  to  escape  the  conviction 
that  these  great  celestial  whorls  are  whirling 
around  an  axis.  And  it  is  most  important  in 
the  study  of  the  growth  of  worlds,  to  recognize 
that  the  type  specimen  is  a  revolving  spiral. 
Therefore,  the  rotating  flattened  globe  of  incan- 
descent matter  postulated  by  Laplace's  nebular 
hypothesis  would  make  of  our  solar  system  an 
exceptional  world,  and  not  a  type  of  stellar  evo- 
lution in  general. 

Keeler's  photographs  have  taught  us  one  thing 
more.  Scarcely  is  there  a  single  one  of  his  nega- 
tives that  does  not  show  nebulae  previously  un- 
catalogued.  It  is  estimated  that  if  this  process  of 
photography  could  be  extended  so  as  to  cover  the 
entire  sky,  the  whole  number  of  nebulae  would 
add  up  to  the  stupendous  total  of  120,000;  and 
of  these  the  great  majority  would  be  spiral. 

When  we  approach  the  question  of  the  distri- 
bution of  nebulae  in  different  parts  of  the  sky,  as 
shown  by  their  catalogued  positions,  we  are  met 
by  a  curious  fact.  It  appears  that  the  region  in 
the  neighborhood  of  the  Milky  Way  is  espe- 

33 


NEBULAE 

cially  poor  in  nebulae,  whereas  these  objects  seem 
to  cluster  in  much  larger  numbers  about  those 
points  in  the  sky  that  are  farthest  from  the 
Milky  Way.  But  we  know  that  the  Milky 
Way  is  richer  in  stars  than  any  other  part  of  the 
sky,  since  it  is,  in  fact,  made  up  of  stellar  bodies 
clustered  so  closely  that  it  is  wellnigh  impossible 
to  see  between  them  in  the  denser  portions. 
Now,  it  cannot  be  the  result  of  chance  that  the 
stars  should  tend  to  congregate  in  the  Milky 
Way,  while  the  nebulae  tend  to  seek  a  position  as 
far  from  it  as  possible.  Whatever  may  be  the 
cause,  we  must  conclude  that  the  sidereal  system, 
as  we  see  it,  is  in  general  constructed  upon  a  sin- 
gle plan,  and  does  not  consist  of  a  series  of  uni- 
verses scattered  at  random  throughout  space.  If 
we  are  to  suppose  that  nebulae  turn  into  stars  as 
a  result  of  condensation  or  any  other  change, 
then  it  is  not  astonishing  to  find  a  minimum  of 
nebulae  where  there  is  a  maximum  of  stars,  since 
the  nebulae  will  have  been  consumed,  as  it  were, 
in  the  formation  of  the  stars. 

It  is   never    advisable   to    push    philosophical 
speculation  very  far  when  supported  by  too  slen- 

34 


The   "Dumb-Bell"    Nebula. 

Photographed   by   Keeler,  July    31,    1899. 
Exposure,  three  hours. 


NEBULAE 

der  a  basis  of  fact.  But  if  we  are  to  regard  the 
visible  universe  as  made  up  on  the  whole  of  a 
single  system  of  bodies,  we  may  well  ask  one  or 
two  questions  to  be  answered  by  speculative  the- 
ory. We  have  said  the  stars  are  not  uniformly 
distributed  in  space.  Their  concentration  in  the 
Milky  Way,  forming  a  narrow  band  dividing  the 
sky  into  two  very  nearly  equal  parts,  must  be 
due  to  their  being  actually  massed  in  a  thin  disk 
or  ring  of  space  within  which  our  solar  system  is 
also  situated.  This  thin  disk  projected  upon  the 
sky  would  then  appear  as  the  narrow  star-band 
of  the  Milky  Way.  Now,  suppose  this  disk  has 
an  axis  perpendicular  to  itself,  and  let  us  imagine 
a  rotation  of  the  whole  sidereal  system  about 
that  axis.  Then  the  fact  that  the  visible  nebu- 
lae are  congregated  far  from  the  Milky  Way 
means  that  they  are  actually  near  the  imaginary 
axis. 

Possibly  the  diminished  velocity  of  motion 
near  the  axis  may  have  something  to  do  with 
the  presence  of  the  nebulae  there.  Possibly  the 
nebulae  themselves  have  axes  perpendicular  to 
the  plane  of  the  Milky  Way.  If  so,  we  should 

35 


NEBULJE 

see  the  spiral  nebulae  near  the  Milky  Way  edge- 
wise, and  those  far  from  it  without  foreshort- 
ening. Thus,  the  paucity  of  nebulae  near  the 
Milky  Way  may  be  due  in  part  to  the  increased 
difficulty  of  seeing  them  when  looked  at  edge- 
wise. Indeed,  there  is  no  limit  to  the  possibil- 
ities of  hypothetical  reasoning  about  the  nebular 
structure  of  our  universe ;  unfortunately,  the 
whole  question  must  be  placed  for  the  present 
among  those  intensely  interesting  cosmic  prob- 
lems awaiting  elucidation,  let  us  hope,  in  this 
new  century. 


TEMPORARY    STARS 

NOTHING  can  be  more  erroneous  than  to  sup- 
pose that  the  stellar  multitude  has  continued  un- 
changed throughout  all  generations  of  men. 
"  Eternal  fires  "  poets  have  called  the  stars  ;  yet 
they  burn  like  any  little  conflagration  on  the 
earth  ;  now  flashing  with  energy,  brilliant,  incan- 
descent, and  again  sinking  into  the  dull  glow  of 
smouldering  half-burned  ashes.  It  is  even  prob- 
able that  space  contains  many  darkened  orbs,  stars 
that  may  have  risen  in  constellations  to  adorn 
the  skies  of  prehistoric  time — now  cold,  unseen, 
unknown.  So  far  from  dealing  with  an  un- 
varying universe,  it  is  safe  to  say  that  sidereal 
astronomy  can  advance  only  by  the  discovery  of 
change.  Observational  science  watches  with  un- 
tiring industry,  and  night  hides  few  celestial  events 
from  the  ardent  scrutiny  of  astronomers.  Old 
theories  are  tested  and  newer  ones  often  perfected 
by  the  detection  of  some  slight  and  previously 

SI 


TEMPORARY   STARS 

unsuspected  alteration  upon  the  face  of  the  sky. 
The  interpretation  of  such  changes  is  the  most 
difficult  task  of  science ;  it  has  taxed  the  acutest 
intellects  among  men  throughout  all  time. 

If,  then,  changes  can  be  seen  among  the  stars, 
what  are  we  to  think  of  the  most  important  change 
of  all,  the  blazing  into  life  of  a  new  stellar  system  ? 
Fifteen  times  since  men  began  to  write  their 
records  of  the  skies  has  the  birth  of  a  star  been 
seen.  Surely  we  may  use  this  term  when  we 
speak  of  the  sudden  appearance  of  a  brilliant  lu- 
minary where  nothing  visible  existed  before.  But 
we  shall  see  further  on  that  scientific  considera- 
tions make  it  highly  probable  that  the  phenom- 
enon in  question  does  not  really  involve  the  crea- 
tion of  new  matter.  It  is  old  material  becoming 
suddenly  luminous  for  some  hidden  reason.  In 
fact,  whenever  a  new  object  of  great  brilliancy  has 
been  discovered,  it  has  been  found  to  lose  its  light 
again  quite  soon,  ending  either  in  total  extinction 
or  at  least  in  comparative  darkness.  It  is  for  this 
reason  that  the  name  "  temporary  star  "  has  been 
applied  to  cases  of  this  kind. 

The  first  authenticated  instance  dates  from  the 
38 


TEMPORARY   STARS 

year  134  B.C.,  when  a  new  star  appeared  in  the 
constellation  Scorpio.  It  was  this  star  that  led 
Hipparchus  to  construct  his  stellar  catalogue,  the 
first  ever  made.  It  occurred  to  him,  of  course, 
that  there  could  be  but  one  way  to  make  sure  in 
the  future  that  any  given  object  discovered  in  the 
sky  was  new ;  it  was  necessary  to  make  a  com- 
plete list  of  everything  visible  in  his  day.  Later 
astronomers  need  then  only  compare  Hippar- 
chus's  catalogue  with  the  heavens  from  time  to 
time  in  order  to  find  out  whether  anything  un- 
known had  appeared.  This  work  of  Hipparchus 
became  the  foundation  of  sidereal  study,  and  led 
to  most  important  discoveries  of  various  kinds. 

But  no  records  remain  concerning  his  new  star 
except  the  bare  fact  of  its  appearance  in  Scorpio. 
Hipparchus's  published  works  are  all  lost.  We 
do  not  even  know  the  exact  place  of  his  birth, 
and  as  for  those  two  dates  of  entry  and  exit  that 
history  attaches  to  great  names — we  have  them 
not.  Yet  he  was  easily  the  first  astronomer  of 
antiquity,  one  of  the  first  of  all  time ;  and  we 
know  of  him  only  from  the  writings  of  Ptolemy, 
who  lived  three  hundred  years  after  him. 

39 


TEMPORARY   STARS 

More  than  five  centuries  elapsed  before  another 
temporary  star  was  entered  in  the  records  of 
astronomy.  This  happened  in  the  year  3 89  A.D., 
when  a  star  appeared  in  Aquila ;  and  of  this  one 
also  we  know  nothing  further.  But  about  twelve 
centuries  later,  in  November,  1572,  a  new  and 
brilliant  object  was  found  in  the  constellation 
Cassiopeia.  It  is  known  as  Tycho's  star,  since  it 
was  the  means  of  winning  for  astronomy  a  man 
who  will  always  take  high  rank  in  her  annals, 
Tycho  Brahe,  of  Denmark.  When  he  first  saw 
this  star,  it  was  already  very  bright,  equalling  even 
Venus  at  her  best ;  and  he  continued  a  careful 
series  of  observations  for  sixteen  months,  when  it 
faded  finally  from  his  view.  The  position  of  the 
new  star  was  measured  with  reference  to  other  stars 
in  the  constellation  Cassiopeia,  and  the  results  of 
Tycho's  observations  were  finally  published  by 
him  in  the  year  1573.  It  appears  that  much  urg- 
ing on  the  part  of  friends  was  necessary  to  induce 
him  to  consent  to  this  publication,  not  because  of 
a  modest  reluctance  to  rush  into  print,  but  for  the 
reason  that  he  considered  it  undignified  for  a  no- 
bleman of  Denmark  to  be  the  author  of  a  book ! 

40 


TEMPORARY   STARS 

An  important  question  in  cosmic  astronomy  is 
opened  by  Tycho's  star.  Did  it  really  disappear 
from  the  heavens  when  he  saw  it  no  more,  or  had 
its  lustre  simply  been  reduced  below  the  visual 
power  of  the  unaided  eye  ?  Unfortunately, 
Tycho's  observations  of  the  star's  position  in  the 
constellation  were  necessarily  crude.  He  pos- 
sessed no  instruments  of  precision  such  as  we  now 
have  at  our  disposal,  and  so  his  work  gives  us 
only  a  rather  rough  approximation  of  the  true 
place  of  the  star.  A  small  circle  might  be  im- 
agined on  the  sky  of  a  size  comparable  with  the 
possible  errors  of  Tycho's  observations.  We 
could  then  say  with  certainty  that  his  star  must 
have  been  situated  somewhere  within  that  little 
circle,  but  it  is  impossible  to  know  exactly  where. 

It  happens  that  our  modern  telescopes  reveal 
the  existence  of  several  faint  stars  within  the 
space  covered  by  such  a  circle.  Any  one  of  these 
would  have  been  too  small  for  Tycho  to  see,  and, 
therefore,  any  one  of  them  may  be  his  once  brilliant 
luminary  reduced  to  a  state  of  permanent  or  tem- 
porary semi-darkness.  These  considerations  are, 
indeed,  of  great  importance  in  explaining  the 

41 


TEMPORARY   STARS 

phenomena  of  temporary  stars.  If  Tycho  had 
been  able  to  leave  us  a  more  exact  determination 
of  his  star's  place  in  the  sky,  and  even  if  our  most 
powerful  instruments  could  not  show  anything  in 
that  place  to-day,  we  might  nevertheless  theorize 
on  the  supposition  that  the  object  still  exists,  but 
has  reached  a  condition  almost  entirely  dark. 

Indeed,  the  latest  theory  classes  temporary  stars 
among  those  known  as  variable.  For  many  stars 
are  known  to  undergo  quite  decided  changes  in 
brilliancy  ;  possibly  inconstancy  of  light  is  the  rule 
rather  than  the  exception.  But  while  such  changes, 
when  they  exist,  are  too  small  to  be  perceptible 
in  most  cases,  there  is  certainly  a  large  number  of 
observable  variables,  subject  to  easily  measurable 
alterations  of  light.  Astronomers  prefer  to  see  in 
the  phenomena  of  temporary  stars  simple  cases  of 
variation  in  which  the  increase  of  light  is  sudden, 
and  followed  by  a  gradual  diminution.  Possibly 
there  is  then  a  long  period  of  comparative  or  even 
complete  darkness,  to  be  followed  as  before  by  a 
sudden  blazing  up  and  extinction.  No  temporary 
star,  however,  has  been  observed  to  reappear  in  the 
same  celestial  place  where  once  had  glowed  its 

42 


TEMPORARY   STARS 

sudden  outburst.  But  cases  are  not  wanting 
where  incandescence  has  been  both  preceded  and 
followed  by  a  continued  existence,  visible  though 
not  brilliant. 

For  such  cases  as  these  it  is  necessary  to  come 
down  to  modern  records.  We  cannot  be  sure 
that  some  faint  star  has  been  temporarily  brilliant, 
unless  we  actually  see  the  conflagration  itself,  or 
are  able  to  make  the  identity  of  the  object's  pre- 
cise location  in  the  sky  before  and  after  the  event 
perfectly  certain  by  the  aid  of  modern  instru- 
ments of  precision.  But  no  one  has  ever  seen 
the  smouldering  fires  break  out.  Temporary 
stars  have  always  been  first  noticed  only  after 
having  been  active  for  hours  if  not  for  days.  So 
we  must  perforce  fall  back  on  instrumental  iden- 
tification by  determinations  of  the  star's  exact 
position  upon  the  celestial  vault. 

Some  time  between  May  loth  and  I2th  in  the 
year  1866  the  ninth  star  in  the  list  of  known 
"  temporaries  "  appeared.  It  possessed  very 
great  light-giving  power,  being  surpassed  in  brill- 
iancy by  only  about  a  score  of  stars  in  all  the 
heavens.  It  retained  a  maximum  luminosity  only 

43 


TEMPORARY   STARS 

three  or  four  days,  and  in  less  than  two  months 
had  diminished  to  a  point  somewhere  between  the 
ninth  and  tenth  "magnitudes."  In  other  words, 
from  a  conspicuous  star,  visible  to  the  naked  eye, 
it  had  passed  beyond  the  power  of  anything  less 
than  a  good  telescope.  Fortunately,  we  had  ex- 
cellent star-catalogues  before  1866.  These  were 
at  once  searched,  and  it  was  possible  to  settle 
quite  definitely  that  a  star  of  about  the  ninth  or 
tenth  magnitude  had  really  existed  before  1866 
at  precisely  the  same  point  occupied  by  the  new 
one.  Needless  to  say,  observations  were  made  of 
the  new  star  itself,  and  afterward  compared  with 
later  observations  of  the  faint  one  that  still  occu- 
pies its  place.  These  render  quite  certain  the 
identity  of  the  temporary  bright  star  with  the 
faint  ones  that  preceded  and  followed  it. 

Such  results,  on  the  one  hand,  offer  an  excel- 
lent vindication  of  the  painstaking  labor  expended 
on  the  construction  of  star-catalogues,  and,  on  the 
other,  serve  to  elucidate  the  mystery  of  tempo- 
rary stars.  Nothing  can  be  more  plausible  than 
to  explain  by  analogy  those  cases  in  which  no 
previous  or  subsequent  existence  has  been  ob- 

44 


TEMPORARY    STARS 

served.  It  is  merely  necessary  to  suppose  that, 
instead  of  varying  from  the  ninth  or  tenth  mag- 
nitude, other  temporary  objects  have  begun  and 
ended  with  the  twentieth ;  for  the  twentieth  mag- 
nitude would  be  beyond  the  power  of  our  best 
instruments. 

Nor  is  the  star  of  1866  an  isolated  instance. 
Ten  years  later,  in  1876,  a  temporary  star  blazed 
up  to  about  the  second  magnitude,  and  returned 
to  invisibility,  so  far  as  the  naked  eye  is  concerned, 
within  a  month,  having  retained  its  greatest  brill- 
iancy only  one  or  two  days.  This  star  is  still 
visible  as  a  tiny  point  of  light,  estimated  to  be  of 
the  fifteenth  magnitude.  Whether  it  existed 
prior  to  its  sudden  outburst  can  never  be  known, 
because  we  do  not  possess  catalogues  including 
the  generality  of  stars  as  faint  as  this  one  must 
have  been.  But  at  all  events,  the  continued  ex- 
istence of  the  object  helps  to  place  the  temporary 
stars  in  the  class  of  variables. 

The  next  star,  already  mentioned  under  cc  neb- 
ula," was  first  seen  in  1885.  ^  was  m  one  re~ 
spect  the  most  remarkable  of  all,  for  it  appeared 
almost  in  the  centre  of  the  great  nebula  in  the 

45 


TEMPORARY   STARS 

constellation  Andromeda.  It  was  never  very 
bright,  reaching  only  the  sixth  magnitude  or 
thereabouts,  was  observed  during  a  period  of  only 
six  months,  and  at  the  end  of  that  time  had  faded 
beyond  the  reach  of  our  most  powerful  glasses. 
It  is  a  most  impressive  fact  that  this  event  oc- 
curred within  the  nebula.  Whatever  may  be  the 
nature  of  the  explosive  catastrophe  to  which  the 
temporary  stars  owe  their  origin,  we  can  now  say 
with  certainty  that  not  even  those  vast  elemental 
luminous  clouds  men  call  nebulae  are  free  from 
danger. 

The  last  outburst  on  our  records  was  first  no- 
ticed February  22,  1901.  The  star  appeared  in 
the  constellation  Perseus,  and  soon  reached  the 
first  magnitude,  surpassing  almost  every  other 
star  in  the  sky.  It  has  been  especially  remark- 
able in  that  it  has  become  surrounded  by  a  nebu- 
lous mass  in  which  are  several  bright  condensa- 
tions or  nuclei ;  and  these  seem  to  be  in  very 
rapid  motion.  The  star  is  still  under  observa- 
tion (January,  1902). 


46 


GALILEO 

AMONG  the  figures  that  stand  out  sharply 
upon  the  dim  background  of  old-time  science, 
there  is  none  that  excites  a  keener  interest  than 
Galileo.  Most  people  know  him  only  as  a  dis- 
tinguished man  of  learning ;  one  who  carried  on 
a  vigorous  controversy  with  the  Church  on  mat- 
ters scientific.  It  requires  some  little  study, 
some  careful  reading  between  the  lines  of  astro- 
nomical history,  to  gain  acquaintance  with  the 
man  himself.  He  had  a  brilliant,  incisive  wit ; 
was  a  genuine  humorist ;  knew  well  and  loved 
the  amusing  side  of  things ;  and  could  not  often 
forego  a  sarcastic  pleasantry,  or  deny  himself  the 
pleasure  of  argument.  Yet  it  is  more  than 
doubtful  if  he  ever  intended  impertinence,  or 
gave  willingly  any  cause  of  quarrel  to  the 
Church. 

His  acute  understanding  must  have  seen  that 
there  exists  no  real  conflict  between  science  and 

47 


GALILEO 

religion ;  for  time,  in  passing,  has  made  common 
knowledge  of  this  truth,  as  it  has  of  many  things 
once  hidden.  When  we  consider  events  that  oc-  . 
curred  three  centuries  ago,  it  is  easy  to  replace 
excited  argument  with  cool  judgment ;  to  re- 
member that  those  were  days  of  violence  and 
cruelty  ;  that  public  ignorance  was  of  a  density 
difficult  to  imagine  to-day ;  and  that  it  was  uni- 
versally considered  the  duty  of  the  Church  to 
assume  an  authoritative  attitude  upon  many 
questions  with  which  she  is  not  now  required  to 
concern  herself  in  the  least.  Charlatans,  unbal- 
anced theorists,  purveyors  of  scientific  marvels, 
were  all  liable  to  be  passed  upon  definitely  by 
the  Church,  not  in  a  spirit  of  impertinent  inter- 
ference, but  simply  as  part  of  her  regular  duties. 
If  the  Church's  judgment  in  such  matters  was 
sometimes  erroneous ;  if  her  interference  now 
and  again  was  cruel,  the  cause  must  be  sought  in 
the  manners  and  customs  of  the  time,  when  per- 
secution rioted  in  company  with  ignorance,  and 
violence  was  the  law.  Perhaps  even  to-day  it 
would  not  be  amiss  to  have  a  modern  scientific 
board  pass  authoritatively  upon  novel  discov- 

48 


GALILEO 

cries  and  inventions,  so  as  to  protect  the  public 
against  impostors  as  the  Church  tried  to  do  of 
old. 

Galileo  was  born  at  Pisa  in  1564,  and  his  long 
life  lasted  until  1642,  the  very  year  of  Newton's 
birth.  His  most  important  scientific  discoveries 
may  be  summed  up  in  a  few  words ;  he  was  the 
first  to  use  a  telescope  for  examining  the  heav- 
enly bodies ;  he  discovered  mountains  on  the 
moon ;  the  satellites  of  Jupiter ;  the  peculiar 
appearance  of  Saturn  which  Huygens  afterward 
explained  as  a  ring  surrounding  the  ball  of  the 
planet ;  and,  finally,  he  found  black  spots  on  the 
sun's  disk.  These  discoveries,  together  with  his 
remarkable  researches  in  mechanical  science,  con- 
stitute Galileo's  claim  to  immortality  as  an  in- 
vestigator. But,  as  we  have  said,  it  is  not  our 
intention  to  consider  his  work  as  a  series  of  sci- 
entific discoveries.  We  shall  take  a  more  inter- 
esting point  of  view,  and  deal  with  him  rather  as 
a  human  being  who  had  contracted  the  habit  of 
making  scientific  researches. 

What  must  have  been  his  feelings  when  he 
first  found  with  his  "  new  "  telescope  the  satel- 

49 


GALILEO 

lites  of  Jupiter  ?  They  were  seen  on  the  night 
of  January  7,  1610.  He  had  already  viewed  the 
planet  through  his  earlier  and  less  powerful 
glass,  and  was  aware  that  it  possessed  a  round 
disk  like  the  moon,  only  smaller.  Now  he  saw 
also  three  objects  that  he  took  to  be  little  stars 
near  the  planet.  But  on  the  following  night,  as 
he  says,  "  drawn  by  what  fate  I  know  not,"  the 
tube  was  again  turned  upon  the  planet.  The 
three  small  stars  had  changed  their  positions,  and 
were  now  all  situated  to  the  west  of  Jupiter, 
whereas  on  the  previous  night  two  had  been  on 
the  eastern  side.  He  could  not  explain  this 
phenomenon,  but  he  recognized  that  there  was 
something  peculiar  at  work.  Long  afterward,  in 
one  of  his  later  works,  translated  into  quaint  old 
English  by  Salusbury,  he  declared  that  "  one  sole 
experiment  sufficeth  to  batter  to  the  ground  a 
thousand  probable  Arguments."  This  was  al- 
ready the  guiding  principle  of  his  scientific  activ- 
ity, a  principle  of  incomparable  importance,  and 
generally  credited  to  Bacon.  Needless  to  say, 
Jupiter  was  now  examined  every  night. 

The  9th  was  cloudy,  but  on  the  loth  he  again 
50 


GALILEO 

saw  his  little  stars,  their  number  now  reduced  to 
two.  He  guessed  that  the  third  was  behind  the 
planet's  disk.  The  position  of  the  two  visible 
ones  was  altogether  different  from  either  of  the 
previous  observations.  On  the  nth  he  became 
sure  that  what  he  saw  was  really  a  series  of  sat- 
ellites accompanying  Jupiter  on  his  journey 
through  space,  and  at  the  same  time  revolving 
around  him.  On  the  I2th,  at  3  A.M.,  he  actu- 
ally saw  one  of  the  small  objects  emerge  from 
behind  the  planet;  and  on  the  i3th  he  finally 
saw  four  satellites.  Two  hundred  and  eighty- 
two  years  were  destined  to  pass  away  before  any 
human  eye  should  see  a  fifth.  It  was  Barnard  in 
1892  who  followed  Galileo. 

To  understand  the  effect  of  this  discovery 
upon  Galileo  requires  a  person  who  has  himself 
watched  the  stars,  not,  as  a  dilettante,  seeking 
recreation  or  amusement,  but  with  that  deep  rev- 
erence that  comes  only  to  him  who  feels — nay, 
knows — that  in  the  moment  of  observation  just 
passed  he  too  has  added  his  mite  to  the  great 
fund  of  human  knowledge.  Galileo's  mummied 
forefinger  still  points  toward  the  stars  from  its 

51 


GALILEO 

little  pedestal  of  wood  in  the  Museo  at  Florence, 
a  sign  to  all  men  that  he  is  unforgotten.  But 
Galileo  knew  on  that  nth  of  January,  1610,  that 
the  memory  of  him  would  never  fade  ;  that  the 
very  music  of  the  spheres  would  thenceforward 
be  attuned  to  a  truer  note,  if  any  would  but 
hearken  to  the  Jovian  harmony.  For  he  recog- 
nized at  once  that  the  visible  revolution  of  these 
moons  around  Jupiter,  while  that  planet  was 
himself  visibly  travelling  through  space,  must 
deal  its  death-blow  to  the  old  Ptolemaic  system 
of  the  universe.  Here  was  a  great  planet,  the 
centre  of  a  system  of  satellites,  and  yet  not 
the  centre  of  the  universe.  Surely,  then,  the 
earth,  too,  might  be  a  mere  planet  like  Jupiter, 
and  not  the  supposed  motionless  centre  of  all 
things. 

The  satellite  discovery  was  published  in  1610 
in  a  little  book  called  cc  Sidereus  Nuncius,"  usually 
translated  c<  The  Sidereal  Messenger."  It  seems 
to  us,  however,  that  the  word  "  messenger  "  is 
not  strong  enough ;  surely  in  Papal  Italy  a  nun- 
cius  was  more  than  a  mere  messenger.  He  was 
clothed  with  the  very  highest  authority,  and  we 

52 


GALILEO 

think  it  probable  that  Galileo's  choice  of  this 
word  in  the  title  of  his  book  means  that  he 
claimed  for  himself  similar  authority  in  science. 
At  all  events,  the  book  made  him  at  once  a  great 
reputation  and  numerous  enemies. 

But  it  was  not  until  1616  that  the  Holy  Office 
(Inquisition)  issued  an  edict  ordering  Galileo  to 
abandon  his  opinion  that  the  earth  moved,  and  at 
the  same  time  placed  Copernicus's  De  Revolutioni- 
bus  and  two  other  books  advocating  that  doc- 
trine on  the  "  Index  Librorum  Prohibitorum," 
or  list  of  books  forbidden  by  the  Church.  These 
volumes  remained  in  subsequent  editions  of  the 
"  Index"  down  to  1821,  but  they  no  longer 
appear  in  the  edition  in  force  to-day. 

Galileo's  most  characteristic  work  is  entitled 
the  "  Dialogue  on  the  Two  Chief  Systems  of  the 
World."  It  was  not  published  until  1632, 
although  the  idea  of  the  book  was  conceived 
many  years  earlier.  I  nit  he  gave  full  play  to  his 
extraordinary  powers  as  a  true  humorist,  a  fine 
lame  among  controversialists,  and  a  genuine 
man  of  science,  valuing  naked  truth  above  all 
other  things.  As  may  be  imagined,  it  was  no 

53 


GALILEO 

small  matter  to  obtain  the  authorities'  consent  to 
this  publication.  Galileo  was  already  known  to 
hold  heretical  opinions,  and  it  was  suspected  that 
he  had  not  laid  them  aside  when  commanded  to 
do  so  by  the  edict  of  1616.  But  perhaps  Gali- 
leo's introduction  to  the  "  Dialogue  "  secured  the 
censor's  imprimatur;  it  is  even  suspected  that 
the  Roman  authorities  helped  in  the  preparation 
of  this  introduction.  Fortunately,  we  have  a  de- 
lightful contemporary  translation  into  English, 
by  Thomas  Salusbury,  printed  at  London  by 
Leybourne  in  1661.  We  have  already  quoted 
from  this  translation,  and  now  add  from  the  same 
work  part  of  Galileo's  masterly  preface  to  the 
"  Dialogue  " : 

"Judicious  reader,  there  was  published  some 
years  since  in  Rome  a  salutiferous  Edict,  that,  for 
the  obviating  of  the  dangerous  Scandals  of  the 
Present  Age,  imposed  a  reasonable  Silence  upon 
the  Pythagorean  (Copernican)  opinion  of  the 
Mobility  of  the  Earth.  There  want  not  such 
as  unadvisedly  affirm,  that  the  Decree  was  not 
the  production  of  a  sober  Scrutiny,  but  of  an 
ill-formed  passion ;  and  one  may  hear  some 

54 


GALILEO 

mutter  that  Consultors  altogether  ignorant  of 
Astronomical  observations  ought  not  to  clipp 
the  wings  of  speculative  wits  with  rash  prohi- 
bitions." 

Galileo  first  states  his  own  views,  and  then 
pretends  that  he  will  oppose  them.  He  goes  on 
to  say  that  he  believes  in  the  earth's  immobility, 
and  takes  "  the  contrary  only  for  a  mathematical 
C&priccio"  as  he  calls  it ;  something  to  be  con- 
sidered, because  possessing  an  academical  interest, 
but  on  no  account  having  a  real  existence.  Of 
course  any  one  (even  a  censor)  ought  to  be  able 
to  see  that  it  is  the  Capriccio,  and  not  its  oppo- 
site, that  Galileo  really  advocates.  Three  persons 
appear  in  the  "  Dialogue  "  :  Salviati,  who  be- 
lieves in  the  Copernican  system;  Simplicio,  of 
suggestive  name,  who  thinks  the  earth  cannot 
move ;  and,  finally,  Sagredus,  a  neutral  gentle- 
man of  humorous  propensities,  who  usually  be- 
gins by  opposing  Salviati,  but  ends  by  being  con- 
vinced. He  then  helps  to  punish  poor  Simplicio, 
who  is  one  of  those  persons  apparently  incapable 
of  comprehending  a  reasonable  argument.  Here 
is  an  interesting  specimen  of  the  "  Dialogue  " 

55 


GALILEO 

taken  from  Salisbury's  translation  :  Salviati  refers 
to  the  argument,  then  well  known,  that  the  earth 
cannot  rotate  on  its  axis,  "  because  of  the  impos- 
sibility of  its  moving  long  without  wearinesse." 
Sagredus  replies  :  "  There  are  some  kinds  of 
animals  which  refresh  themselves  after  wearinesse 
by  rowling  on  the  earth  ;  and  that  therefore  there 
is  no  need  to  fear  that  the  Terrestrial  Globe 
should  tire,  nay,  it  may  be  reasonably  affirmed 
that  it  enjoy eth  a  perpetual  and  most  tranquil  re- 
pose, keeping  itself  in  an  eternal  rowling."  Sal- 
viati's  comment  on  this  sally  is,  "  You  are  too 
tart  and  satyrical,  Sagredus." 

There  is  no  doubt  that  the  "  Dialogue  "  fin- 
ished the  Ptolemaic  theory,  and  made  that  of 
Copernicus  the  only  possible  one.  At  all  events, 
it  brought  about  the  well-known  attack  upon 
Galileo  from  the  authorities  of  the  Holy  Office. 
We  shall  not  recount  the  often-told  tale  of  his 
recantation.  He  was  convicted  (very  rightly)  of 
being  a  Copernican,  and  was  forced  to  abjure  that 
doctrine.  Galileo's  life  may  be  summed  up  as 
one  of  those  through  which  the  world  has  been 
made  richer.  A  clean-cutting  analytic  wit,  never 

56 


GALILEO 

becoming  dull :  heated  again  and  again  in  the 
fierce  blaze  of  controversy,  it  was  allowed  to  cool 
only  that  it  might  acquire  a  finer  temper,  to 
pierce  with  fatal  certainty  the  smallest  imperfec- 
tions in  the  armor  of  his  adversaries. 


57 


THE    PLANET    OF    1898 

THE  discovery  of  a  new  and  important  planet 
usually  receives  more  immediate  popular  atten- 
tion and  applause  than  any  other  astronomical 
event.  Philosophers  are  fond  of  referring  to  our 
solar  system  as  a  mere  atom  among  the  countless 
universes  that  seem  to  be  suspended  within  the 
profound  depths  of  space.  They  are  wont  to 
point  out  that  this  solar  system,  small  and  in- 
significant as  a  whole  in  comparison  with  many 
of  the  stellar  worlds,  is,  nevertheless,  made  up  of 
a  large  number  of  constituent  planets ;  and  these 
in  turn  are  often  accompanied  with  still  smaller 
satellites,  or  moons.  Thus  does  Nature  provide 
worlds  within  worlds,  and  it  is  not  surprising 
that  public  attention  should  be  at  once  attracted 
by  any  new  member  of  our  sun's  own  special 
family  of  planets.  The  ancients  were  acquainted 
with  only  five  of  the  bodies  now  counted  as 
planets,  viz. :  Mercury,  Venus,  Mars,  Jupiter, 

58 


THE   PLANET   OF    1898 

and  Saturn.  The  dates  of  their  discovery  are 
lost  in  antiquity.  To  these  Uranus  was  added 
in  1781  by  a  brilliant  effort  of  the  elder  Herschel. 
We  are  told  that  intense  popular  excitement 
followed  the  announcement  of  Herschel's  first 
observation :  he  was  knighted  and  otherwise 
honored  by  the  English  King,  and  was  enabled 
to  lay  a  secure  foundation  for  the  future  distin- 
guished astronomical  reputation  of  his  family. 

Herschel's  discovery  quickened  the  restless 
activity  of  astronomers.  Persistent  efforts  were 
made  to  sift  the  heavens  more  and  more  closely, 
with  the  strengthened  hope  of  adding  still  further 
to  our  planetary  knowledge.  An  association 
of  twenty-four  enthusiastic  German  astronomers 
was  formed  for  the  express  purpose  of  hunt- 
ing planets.  But  it  fell  to  the  lot  of  an  Ital- 
ian, Piazzi,  of  Palermo,  to  find  the  first  of  that 
series  of  small  bodies  now  known  as  the  asteroids 
or  minor  planets.  He  made  the  discovery  at  the 
very  beginning  of  our  century,  January  i,  1801. 

But  news  travelled  slowly  in  those  days,  and  it 
was  not  until  nearly  April  that  the  German  observ- 
ers heard  from  Piazzi.  In  the  meantime,  he  had 

59 


THE   PLANET   OF    1898 

himself  been  prevented  by  illness  from  continu- 
ing his  observations.  Unfortunately,  the  planet 
had  by  this  time  moved  so  near  the  sun,  on  ac- 
count of  its  own  motions  and  those  of  the  earth, 
that  it  could  no  longer  be  observed.  The  bright 
light  of  the  sun  made  observations  of  the  new 
body  impossible ;  and  it  was  feared  that,  owing 
to  lack  of  knowledge  of  the  planet's  orbit,  astron- 
omers would  be  unable  -to  trace  it.  So  there 
seemed,  indeed,  to  be  danger  of  an  almost  irrep- 
arable loss  to  science.  But  in  scientific,  as  in 
other  human  emergencies,  someone  always  ap- 
pears at  the  proper  moment.  A  very  young 
mathematician  at  Gottingen,  named  Gauss,  at- 
tacked the  problem,  and  was  able  to  devise  a 
method  of  predicting  the  future  course  of  the 
planet  on  the  sky,  using  only  the  few  observa- 
tions made  by  Piazzi  himself.  Up  to  that  time 
no  one  had  attempted  to  compute  a  planetary 
orbit,  unless  he  had  at  his  disposal  a  series  of 
observations  extending  throughout  the  whole 
period  of  the  planet's  revolution  around  the  sun. 
But  the  Piazzi  planet  offered  a  new  problem  in 
astronomy.  It  had  become  imperatively  neces- 


60 


THE   PLANET   OF    1898 

sary  to  obtain  an  orbit  from  a  few  observations 
made  at  nearly  the  same  date.  Gauss's  work  was 
signally  triumphant,  for  the  planet  was  actually 
found  in  the  position  predicted  by  him,  as  soon 
as  a  change  in  the  relative  places  of  the  planet 
and  earth  permitted  suitable  observations  to  be 
made. 

But  after  all,  Piazzi's  planet  belongs  to  a  class 
of  quite  small  bodies,  and  is  by  no  means  as  in- 
teresting as  Herschel's  discovery,  Uranus.  Yet 
even  this  must  be  relegated  to  second  rank 
among  planetary  discoveries.  On  September  23, 
1846,  the  telescope  of  the  Berlin  Observatory 
was  directed  to  a  certain  point  on  the  sky  for  a 
very  special  reason.  Galle,  the  astronomer  of 
Berlin,  had  received  a  letter  from  Leverrier,  of 
Paris,  telling  him  that  if  he  would  look  in  a  certain 
direction  he  would  detect  a  new  and  large  planet. 

Leverrier's  information  was  based  upon  a  math- 
ematical calculation.  Seated  in  his  study,  with 
no  instruments  but  pen  and  paper,  he  had  slowly 
figured  out  the  history  of  a  world  as  yet  un- 
seen. Tiny  discrepancies  existed  in  the  observed 
motions  of  Herschel's  planet  Uranus.  No  man 

61 


THE   PLANET  OF    1898 

had  explained  their  cause.  To  Leverrier's  acute 
understanding  they  slowly  shaped  themselves 
into  the  possible  effects  of  attraction  emanating 
from  some  unknown  planet  exterior  to  Uranus. 
Was  it  conceivable  that  these  slight  tremulous 
imperfections  in  the  motion  of  a  planet  could  be 
explained  in  this  way  ?  Leverrier  was  able  to 
say  confidently,  "  Yes."  But  we  may  rest  as- 
sured that  Galle  had  but  small  hopes  that  upon 
his  eye  first,  of  all  the  myriad  eyes  of  men,  would 
fall  a  ray  of  the  new  planet's  light.  Careful 
and  methodical,  he  would  neglect  no  chance  of 
advancing  his  beloved  science.  He  would  look. 
Only  one  who  has  himself  often  seen  the  morn- 
ing's sunrise  put  an  end  to  a  night's  observation 
of  the  stars  can  hope  to  appreciate  what  Galle's 
feelings  must  have  been  when  he  saw  the  planet. 
To  his  trained  eye  it  was  certainly  recognizable 
at  once.  And  then  the  good  news  was  sent  on 
to  Paris.  We  can  imagine  Leverrier,  the  cool 
calculator,  saying  to  himself:  "  Of  course  he 
found  it.  It  was  a  mathematical  certainty." 
Nevertheless,  his  satisfaction  must  have  been 
of  the  keenest.  No  triumphs  give  a  pleasure 


62 


THE   PLANET   OF    1898 

higher  than  those  of  the  intellect.  Let  no  one 
imagine  that  men  who  make  researches  in  the 
domain  of  pure  science  are  under-paid.  They 
find  their  reward  in  pleasure  that  is  beyond  any 
price. 

The  Leverrier  planet  was  found  to  be  the  last 
of  the  so-called  major  planets,  so  far  as  we  can 
say  in  the  present  state  of  science.  It  received 
the  name  Neptune.  Observers  have  found  no 
other  member  of  the  solar  system  comparable  in 
size  with  such  bodies  as  Uranus  and  Neptune. 
More  than  one  eager  mathematician  has  tried  to 
repeat  Leverrier's  achievement,  but  the  supposed 
planet  was  not  found.  It  has  been  said  that  fig- 
ures never  lie ;  yet  such  is  the  case  only  when 
the  computations  are  correctly  made.  People 
are  prone  to  give  to  the  work  of  careless  or  in- 
competent mathematicians  the  same  degree  of 
credence  that  is  really  due  only  to  masters  of  the 
craft.  It  requires  the  test  of  time  to  affix  to  any 
man's  work  the  stamp  of  true  genius. 

While,  then,  we  have  found  no  more  large 
planets,  quite  a  group  of  companions  to  Piazzi's 
little  one  have  been  discovered.  They  are  all 

63 


THE   PLANET   OF    1898 

small,  probably  never  exceeding  about  400  miles 
in  diameter.  All  travel  around  the  sun  in  orbits 
that  lie  wholly  within  that  of  Jupiter  and  are  ex- 
terior to  that  of  Mars.  The  introduction  of  as- 
tronomical photography  has  given  a  tremendous 
impetus  to  the  discovery  of  these  minor  planets, 
as  they  are  called.  It  is  quite  interesting  to  ex- 
amine the  photographic  process  by  which  such 
discoveries  are  made  possible  and  even  easy. 
The  matter  will  not  be  difficult  to  understand  if 
we  remember  that  all  the  planets  are  continually 
changing  their  places  among  the  other  stars.  For 
the  planets  travel  around  the  sun  at  a  compara- 
tively small  distance.  The  great  majority  of  the 
stars,  on  the  contrary,  are  separated  from  the  sun 
by  an  almost  immeasurable  space.  As  a  result, 
they  do  not  seem  to  move  at  all  among  them- 
selves, and  so  we  call  them  fixed  stars  :  they  may, 
indeed,  be  in  motion,  but  their  great  distance  pre- 
vents our  detecting  it  in  a  short  period  of  time. 

Now,  stellar  photographs  are  made  in  much 
the  same  way  as  ordinary  portraits.  Only,  in- 
stead of  using  a  simple  camera,  the  astronomer 
exposes  his  photographic  plate  at  the  eye-end  of 

64 


THE   PLANET   OF    i 

a  telescope.  The  sensitive  surface  of  the  plate  is 
substituted  for  the  human  eye.  We  then  find 
on  the  picture  a  little  dot  corresponding  to  every 
star  within  the  photographed  region  of  the  sky. 
But,  as  everyone  knows,  the  turning  of  the 
earth  on  its  axis  makes  the  whole  heavens,  in- 
cluding the  sun,  moon,  and  stars,  rise  and  set 
every  day.  So  the  stars,  when  we  photograph 
them,  are  sure  to  be  either  climbing  up  in  the 
eastern  sky  or  else  slowly  creeping  down  in  the 
western.  And  that  makes  astronomical  photog- 
raphy very  different  from  ordinary  portrait  work. 
The  stars  correspond  to  the  sitter,  but  they 
don't  sit  still.  For  this  reason  it  is  necessary  to 
connect  the  telescope  with  a  mechanical  contriv- 
ance which  makes  it  turn  round  like  the  hour- 
hand  of  an  ordinary  clock.  The  arrangement  is 
so  adjusted  that  the  telescope,  once  aimed  at  the 
proper  object  in  the  sky,  will  move  so  as  to  re- 
main pointed  exactly  the  same  during  the  whole 
time  of  the  photographic  exposure.  Thus,  while 
the  light  of  any  star  is  acting  on  the  plate,  such 
action  will  be  continuous  at  a  single  point. 
Consequently,  the  finished  picture  will  show  the 

65 


THE   PLANET   OF    1898 

star  as  a  little  dot;  while  without  this  arrange- 
ment, the  star  would  trail  out  into  a  line  in- 
stead of  a  dot.  Now  we  have  seen  that  the 
planets  are  all  moving  slowly  among  the  fixed 
stars.  So  if  we  make  a  star  photograph  in  a  part 
of  the  sky  where  a  planet  happens  to  be,  the 
planet  will  make  a  short  line  on  the  plate ; 
whereas,  if  the  planet  remained  quite  unmoved 
relatively  to  the  stars  it  would  give  a  dot  like 
the  star  dots.  The  presence  of  a  line,  therefore, 
at  once  indicates  a  planet. 

This  method  of  planet-hunting  has  proved 
most  useful.  More  than  400  small  planets  sim- 
ilar to  Piazzi's  have  been  found,  though  never 
another  one  like  Uranus  and  Neptune.  As 
we  have  said,  all  these  little  bodies  lie  between 
Mars  and  Jupiter.  They  evidently  belong  to  a 
group  or  family,  and  many  astronomers  have 
been  led  to  believe  that  they  are  but  fragments 
of  a  former  large  planet. 

In  August,  1898,  however,  one  was  found  by 
Witt,  of  Berlin,  which  will  probably  occupy  a 
very  prominent  place  in  the  annals  of  astronomy. 
For  this  planet  goes  well  within  the  orbit  of 

66 


THE   PLANET   OF    1898 

Mars,  and  this  will  bring  it  at  times  very  close 
to  the  earth.  In  fact,  when  the  motions  of  the 
new  planet  and  the  earth  combine  to  bring  them 
to  their  positions  of  greatest  proximity,  the  new 
planet  will  approach  us  closer  than  any  other 
celestial  body  except  our  own  moon.  Witt 
named  his  new  planet  Eros.  Its  size,  though 
small,  may  prove  to  be  sufficient  to  bring  it  within 
the  possibilities  of  naked-eye  observation  at  the 
time  of  closest  approach  to  the  earth. 

To  astronomers  the  great  importance  of  this 
new  planet  is  due  to  the  following  circumstance : 
For  certain  reasons  too  technical  to  be  stated 
here  in  detail,  the  distance  from  the  earth  to  any 
planet  can  be  determined  with  a  degree  of  pre- 
cision which  is  greatest  for  planets  that  are  near 
us.  Thus  in  time  we  shall  learn  the  distance  of 
Eros  more  accurately  than  we  know  any  other 
celestial  distance.  From  this,  by  a  process  of 
calculation,  the  solar  distance  from  the  earth  is 
determinable.  But  the  distance  from  earth  to 
sun  is  the  fundamental  astronomical  unit  of  meas- 
ure ;  so  that  Witt's  discovery,  through  its  effect 
on  the  unit  of  measure,  will  doubtless  influence 


THE   PLANET   OF    1898 

every  part  of  the  science  of  astronomy.  Here 
we  have  once  more  a  striking  instance  of  the 
reward  sure  to  overtake  the  diligent  worker  in 
science — a  whole  generation  of  men  will  doubt- 
less pass  away  before  we  shall  have  exhausted 
the  scientific  advantages  to  be  drawn  from  Witt's 
remarkable  observation  of  1898. 


68 


HOW    TO    MAKE    A   SUN-DIAL* 

LONG  before  clocks  and  watches  had  been  in- 
vented, people  began  to  measure  time  with  sun- 
dials. Nowadays,  when  almost  everyone  has  a 
watch  in  his  pocket,  and  can  have  a  clock,  too, 
on  the  mantel-piece  of  every  room  in  the  house, 
the  sun-dial  has  ceased  to  be  needed  in  ordinary 
life.  But  it  is  still  just  as  interesting  as  ever  to 
anyone  who  would  like  to  have  the  means  of 
getting  time  direct  from  the  sun,  the  great  hour- 
hand  or  timekeeper  of  the  sky.  Any  person 
who  is  handy  with  tools  can  make  a  sun-dial 
quite  easily,  by  following  the  directions  given 
below. 

In  the  first  place,  you  must  know  that  the 
sun-dial  gives  the  time  by  means  of  the  sun's 
shadow.  If  you  stick  a  walking-cane  up  in  the 
sand  on  a  bright,  sunshiny  day,  the  cane  has  a 

*  This  chapter  is  especially  intended  for  boys  and  girls  and  others  who  like 
to  make  things  with  carpenters'  tools. 

69 


HOW    TO   MAKE   A   SUN-DIAL 

long  shadow  that  looks  like  a  dark  line  on  the 
ground.  Now  if  you  watch  this  shadow  care- 
fully, you  will  see  that  it  does  not  stay  in  the 
same  place  all  day.  Slowly  but  surely,  as  the 
sun  climbs  up  in  the  sky,  the  shadow  creeps 
around  the  cane.  You  can  see  quite  easily  that 
if  the  cane  were  fastened  in  a  board  floor,  and  if 
we  could  mark  on  the  floor  the  p^ces  where  the 
shadow  was  at  different  hours  of  the  day,  we 
could  make  the  shadow  tell  us  the  time  just  like 
the  hour-hand  of  a  clock.  A  sun-dial  is  just 
such  an  arrangement  as  this,  and  I  will  show  you 
how  to  mark  the  shadow  places  exactly,  so  as  to 
tell  the  right  time  without  any  trouble  whenever 
the  sun  shines. 

If  you  were  to  watch  very  carefully  such  an 
arrangement  as  a  cane  standing  in  a  board  floor, 
you  would  not  find  the  creeping  shadow  in  just 
the  same  place  at  the  same  time  every  day.  If 
you  marked  the  place  of  the  shadow  at  exactly 
ten  o'clock  by  your  watch  some  morning,  and 
then  went  back  another  day  at  ten,  you  would 
not  find  the  shadow  on  the  old  mark.  It  would 
not  get  very  far  from  it  in  a  day  or  two,  but  in  a 

70 


HOW   TO   MAKE  A  SUN-DIAL 

month  or  so  it  would  be  quite  a  distance  away. 
Now,  of  course,  a  sun-dial  would  be  of  no  use  if 
it  did  not  tell  the  time  correctly  every  day  ;  and 
in  fact,  it  is  not  easy  to  make  a  dial  when  the 
shadow  is  cast  by  a  stick  standing  straight  up. 
But  we  can  get  over  this  difficulty  very  well  by 
letting  the  shadow  be  cast  by  a  stick  that  leans 
over  toward  th%  floor  just  the  right  amount,  as  I 
will  explain  in  a  moment.  Of  course,  we  should 
not  really  use  the  floor  for  our  sun-dial.  It  is 
much  better  to  mark  out  the  hour-lines,  as  they 
are  called,  on  a  smooth  piece  of  ordinary  white 
board,  and  then,  after  the  dial  is  finished,  it  can 
be  screwed  down  to  a  piazza  floor  or  railing,  or  it 
can  be  fastened  on  a  window-sill.  It  ought  to 
be  put  in  a  place  where  the  sun  can  get  at  it 
most  of  the  time,  because,  of  course,  you  cannot 
use  the  sun-dial  when  the  sun  is  not  shining  on 
it.  If  the  dial  is  set  on  a  window-sill  (of  a  city 
house,  for  instance)  you  must  choose  a  south 
window  if  you  can,  so  as  to  get  the  sun  nearly  all 
day.  If  you  have  to  take  an  east  window,  you 
can  use  the  dial  in  the  morning  only,  and  in  a 
west  window  only  in  the  afternoon.  Sometimes 

71 


HOW   TO   MAKE   A   SUN-DIAL 

it  is  best  not  to  try  to  fasten  the  dial  to  its 
support  with  screws,  but  just  to  mark  its  place, 
and  then  set  it  out  whenever  you  want  to  use 
it.  For  if  the  dial  is  made  of  wood,  and  not 


Fig.   i. 

painted,  it  might  be  injured  by  rain  or  snow 
in  bad  weather  if  left  out  on  a  window-sill  or 
piazza. 

It  is  not  quite  easy  to  fasten  a  little  stick  to  a 
board  so  that  it  will  lean  over  just  right.  So  it 
is  better  not  to  use  a  stick  or  a  cane  in  the  way 
I  have  described,  but  instead  to  use  a  piece  of 
board  cut  to  just  the  right  shape. 

Fig.  i  shows  what  a  sun-dial  should  look  like. 
72 


HOW   TO   MAKE   A   SUN-DIAL 

The  lines  to  show  the  shadow's  place  at  the  dif- 
ferent hours  of  the  day  will  be  marked  on  the 
board  ABCD,  and  this  will 
be  put  flat  on  the  window-sill 
or  piazza  floor.  The  three- 
cornered  piece  of  board  abc 


is  fastened  to  the  bottom-  <* 
board  ABCD  by  screws  going 
through  ABCD  from  underneath.  The  edge  ab 
of  the  three-cornered  board  abc  then  takes  the 
place  of  the  leaning  stick  or  cane,  and  the  time  is 
marked  by  the  shadow  cast  by  the  edge  ab.  Of 
course,  it  is  important  that  this  edge  should  be 
straight  and  perfectly  flat  and  even.  If  you  are 
handy  with  tools,  you  can  make  it  quite  easily, 
but  if  not,  you  can  mark  the  right  shape  on  a 
piece  of  paper  very  carefully,  and  take  it  to  a 
carpenter,  who  can  cut  the  board  according  to 
the  pattern  you  have  marked  on  the  paper. 

Now  I  must  tell  you  how  to  draw  the  shape 
of  the  three-cornered  board  abc.  Fig.  2  shows 
how  it  is  done.  The  side  ac  should  always  be 
just  five  inches  long.  The  side  be  is  drawn  at 
right  angles  to  ac,  which  you  can  do  with  an  or- 

73 


HOW   TO   MAKE   A   SUN-DIAL 


dinary  carpenter's  square.  The  length  of  be  de- 
pends on  the  place  for  which  the  dial  is  made. 
The  following  table  gives  the  length  of  be  for 
various  places  in  the  United  States,  and,  after  you 
have  marked  out  the  length  of  bcy  it  is  only 
necessary  to  complete  the  three-cornered  piece  by 
drawing  the  side  ab  from  a  to  b. 

TABLE  SHOWING  THE  LENGTH  OF  THE  SIDE  be. 


Place. 

be 

Inches. 

Place. 

be 
Inches. 

Albany 

.    .   4  11-16 

New  York   

-.4     v8 

Baltimore             .    . 

..4      -16 

Omaha  

-  .4     3-8 

Boston     

4       -a 

Philadelphia  

47-16 

Buffalo 

4  i  -16 

Pittsburg 

4      3-8 

Charleston 

3-4. 

Portland,   Me 

4   I7-l6 

Chicago 

4       -2. 

Richmond 

•?   ic-i6 

Cincinnati 

4       -16 

Rochester 

4  11-16 

Cleveland 

4        -2 

San  Diego          .  .  . 

-  -  3     i-4 

Denver  

.      ..4      -16 

San  Francisco  

..7  i<;-i6 

Detroit  

4       -2 

Savannah  

.--3     1-8 

Indianapolis 

4.       -16 

St.  Louis 

7  ic-i6 

Kansas    City 

1  1^-16 

St.    Paul 

c 

Louisville 

7     IC-l6 

Seattle. 

.  C       Q-l6 

Milwaukee 

-  -3  11-16 

Washington,    D.    C   .  . 

4     1-16 

New    Orleans.. 

..2       7-8 

If  you  wish  to  make  a  dial  for  a  place  not  given 
in  the  table,  it  will  be  near  enough  to  use  the  dis- 
tance be  as  given  for  the  place  nearest  to  you 
But  in  selecting  the  nearest  place  from  the  table, 
please  remember  to  take  that  one  of  the  cities 
mentioned  which  is  nearest  to  you  in  a  north-and- 

74 


HOW   TO   MAKE  A   SUN-DIAL 

south  direction.  It  does  not  matter  how  far  away 
the  place  is  in  an  east-and-west  direction.  So,  in- 
stead of  taking  the  place  that  is  nearest  to  you  on 
the  map  in  a  straight  line,  take  the  place  to  which 
you  could  travel  by  going  principally  east  or  west, 
and  very  little  north  or  south.  The  figure  drawn 
is  about  the  right  shape  for  New  York.  The 
board  used  for  the  three-cornered  piece  should  be 
about  one-half  inch  thick.  But  if  you  are  making 
a  window-sill  dial,  you  may  prefer  to  have  it 
smaller  than  I  have  described.  You  can  easily 
have  it  half  as  big  by  making  all  the  sizes  and 
lines  in  half-inches  where  the  table  calls  for 
inches. 

After  you  have  marked  out  the  dimensions  for 
the  three-cornered  piece  that  is  to  throw  the 
shadow,  you  can  prepare  the  dial  itself,  with  the 
lines  that  mark  the  place  of  the  shadow  for  every 
hour  of  the  day.  This  you  can  do  in  the  manner 
shown  in  Fig.  3.  Just  as  in  the  case  of  the  three- 
cornered  piece,  you  can  draw  the  dial  with  a  pencil 
directly  on  a  smooth  piece  of  white  board,  about 
three-quarters  of  an  inch  thick,  or  you  can  mark 
it  out  on  a  paper  pattern  and  transfer  it  afterward 

75 


HOW   TO   MAKE  A   SUN-DIAL 


to  the  board.  Perhaps  it  will  be  as  well  to  begin 
by  drawing  on  paper,  as  any  mistakes  can  then 
be  corrected  before  you  commence  to  mark  your 
wood. 

In  the  first  place  you  must  draw  a  couple   of 


X     XI       XII 


IX 


VIII 


VI 


M' 


ti*r*fir 

x*  IN. 


IV 


V! 


lines  MN  and  M'N',  eight  inches  long,  and  just 
far  enough  apart  to  fit  the  edge  of  your  three- 
cornered  shadow-piece.  You  will  remember  I 
told  you  to  make  that  one-half  inch  thick,  so 
your  two  lines  will  also  be  one-half  inch  apart. 
Now  draw  the  two  lines  NO  and  N'O'  square 

76 


HOW   TO   MAKE   A   SUN-DIAL 

with  MN  and  M'N',  and  make  the  distances 
NO  and  N'O'  just  five  inches  each.  The  lines 
OK,  O'K',  and  the  other  lines  forming  the  outer 
border  of  the  dial,  are  then  drawn  just  as  shown, 
OK  and  O'K'  being  just  eight  inches  long,  the 
same  as  MN  and  M'N'.  The  lower  lines  in 
the  figure,  which  are  not  very  important,  are  to 
complete  the  squares.  You  must  mark  the  lines 
NO  and  N'O'  with  the  figures  VI,  these  being 
the  lines  reached  by  the  shadow  at  six  o'clock  in 
the  morning  and  evening.  The  points  where  the 
VII,  VIII,  and .  other  hour-lines  cut  the  lines 
OK,  O'K',  MK,  and  M'K'  can  be  found  from 
the  table  on  page  78. 

In  using  the  table  you  will  notice  that  the  line 
IX  falls  sometimes  on  one  side  of  the  corner  K, 
and  sometimes  on  the  other.  Thus  for  Albany 
the  line  passes  seven  and  seven-sixteenth  inches 
from  O,  while  for  Charleston  it  passes  four  and 
three-eighth  inches  from  M.  For  Baltimore  it 
passes  exactly  through  the  corner  K. 

The  distance  for  the  line  marked  V  from  O'  is 
just  the  same  as  the  distance  from  O  to  VII. 
Similarly,  IV  corresponds  to  VIII,  III  to  IX, 

77 


HOW    TO   MAKE  A   SUN-DIAL 


TABLE  SHOWING  How  TO  MARK  THE  HOUR-LINES. 


PLACE. 

Distance  from  O  to  the  line 
marked 

Distance  from  M  to  the  line 
marked 

VII. 

VIII. 

IX. 

IX. 

X. 

XI. 

Inches. 

Inches. 

Inches. 
7     7~l6 
8 
7    7-16 
7     7-16 

7     7-16 
7     7-16 

Inches. 
4    3-8 

4     1-16 

4    3-8 
4     1-4 

Inches. 
3     1-16 
2     7-8 

!  \~-\l 

2       1-2 

3     1-16 
278 

Inches. 
7-16 
7-16 
7-16 

tf 

7-16 
7-16 
7-16 
7  16 

Baltimore.         .         .    . 

1-8 

15-16 

7-16 

1-8 
1-8 
1-8 

15-16 
11-16 

1-8 
15-16 

15-16 
7-16 

9-16 

4  11-16 
4     5~l6 

t  P6 

4     5-i6 
4  11-16 
4     5-J6 
4     1-2 
4     5-!6 
4  11-16 
4  11-16 
4  11-16 
4    3-i6 
5     3-4 

4     5~l6 
4     1-2 

4     3-i6 
4  11-16 

\  P6 

4  11-16 
5     9~l6 

Buffalo 

Chicago.     .          

•3.    i  16 

Denver. 

2       7-8 

3     1-16 

2     7-8 
2     7-8 
2     7-8 

Detroit 

J  V4  vj  vj  00  00  OO--J  » 

I*  7  7  i  i 

>  ON  ON  ON  ON 

7-16 
7-16 

5  16 

Indianapolis         

Louisville 

3     ,-16 
2     5-1  6 
3     1-16 
3      X-IO 

2     7-8 
3     1-16 
3     3-*6 
2     7-8 
3     1-16 

2       1-2 

2    7-8 

2       1-2 
2       7-8 

3    3—6 

3     3-8 
2     7-8 

7-16 
7-16 
7-16 
7-16 

1-2 

5-J6 
5-x6 

1-2 
1-2 

7-16 

New  Orleans.   .  .  . 

New  York  

Omaha  

Philadelphia 

Pittsburg  

7  11-16 
7     1-8 

7    7-16 
8 

8 
7     1-8 
6    5-8 
8 

Portland,  Me 

Richmond  

Rochester  
San  Diego  

San  Francisco  
Savannah  

St.  Paul 

15-16 
13-16 
1-8 

4     1-16 
3  15-16 
4  11-16 

Seattle  

Washington,  D.  C  

II  to  X,  and  I  to  XL  The  number  XII  is 
marked  at  MM'  as  shown.  If  you  desire  to  add 
lines  (not  shown  in  Fig.  3  to  avoid  confusion)  for 
hours  earlier  than  six  in  the  morning,  it  is  merely 
necessary  to  mark  off  a  distance  on  the  line  KO, 
below  the  point  O,  and  equal  to  the  distance  from 
O  to  VII.  This  will  give  the  point  where  the 

78 


HOW   TO   MAKE   A   SUN-DIAL 

5  A.M.  shadow  line  drawn  from  N  cuts  the  line 
KO.  A  corresponding  line  for  7  P.M.  can  be 
drawn  from  N'  on  the  other  side  of  the  figure. 

After  you  have  marked  out  the  dial  very  care- 
fully, you  must  fasten  the  three-cornered  shadow- 
piece  to  it  in  such  a  way  that  the  whole  instru- 
ment will  look  like  Fig.  i.  The  edge  ac  (Fig. 
2)  goes  on  NM  (Fig.  3).  The  point  a  (Fig.  2) 
must  come  exactly  on  N  (Fig.  3)  ;  and  as  the 
lines  NM  (Fig.  3)  and  N'M'  (Fig.  3)  have  been 
made  just  the  right  distance  apart  to  fit  the 
thickness  of  the  three-cornered  piece  abc  (Fig. 
2),  everything  will  go  together  just  right.  The 
point  c  (Fig.  2)  will  not  quite  reach  to  M  (Fig.  3), 
but  will  be  on  the  line  NM  (Fig.  3)  at  a  distance 
of  three  inches  from  M.  The  two  pieces  of 
wood  will  be  fastened  together  with  three  screws 
going  through  the  bottom-board  ABCD  (Figs. 
i  and  3)  and  into  the  edge  ac  (Fig.  2)  of  the 
three-cornered  piece.  The  whole  instrument  will 
then  look  something  like  Fig.  i. 

After  you  have  got  your  sun-dial  put  together, 
you  need  only  set  it  in  the  sun  in  a  level  place, 
on  a  piazza  or  window-sill,  and  turn  it  round 

79 


HOW   TO   MAKE   A   SUN-DIAL 

until  it  tells  the  right  time  by  the  shadow.  You 
can  get  your  local  time  from  a  watch  near  enough 
for  setting  up  the  dial.  Once  the  dial  is  set  right 
you  can  screw  it  down  or  mark  its  position,  and 
it  will  continue  to  give  correct  solar  time  every 
day  in  the  year. 

If  you  wish  to  adjust  the  dial  very  closely, 
you  must  go  out  some  fine  day  and  note  the  er- 
ror of  the  dial  by  a  watch  at  about  ten  in  the 
morning,  and  at  noon,  and  again  at  about  two  in 
the  afternoon.  If  the  error  is  the  same  each  time, 
the  dial  is  rightly  set.  If  not,  you  must  try,  by 
turning  the  dial  slightly,  to  get  it  so  placed  that 
your  three  errors  will  be  nearly  the  same.  When 
you  have  got  them  as  nearly  alike  as  you  can,  the 
dial  will  be  sufficiently  near  right.  The  solar  or 
dial  time  may,  however,  differ  somewhat  from 
ordinary  watch  time,  but  the  difference  will  never 
be  great  enough  to  matter,  when  we  remember 
that  sun-dials  are  only  rough  timekeepers  after 
all,  and  useful  principally  for  amusement. 


80 


PHOTOGRAPHY    IN    ASTRONOMY 

NEW  highways  of  science  have  been  monu- 
mented  now  and  again  by  the  masterful  efforts  of 
genius,  working  single-handed ;  but  more  often 
it  is  slow-moving  time  that  ripens  discovery,  and, 
at  the  proper  moment,  opens  some  new  path  to 
men  whose  intellectual  power  is  but  willingness 
to  learn.  So  the  annals  of  astronomical  photog- 
raphy do  not  recount  the  achievements  of  extra- 
ordinary genius.  It  would  have  been  strange, 
indeed,  if  the  discovery  of  photography  had  not 
been  followed  by  its  application  to  astronomy. 

The  whole  range  of  chemical  science  contains 
no  experiment  of  greater  inherent  interest  than 
the  development  of  a  photographic  plate.  Let 
but  the  smallest  ray  of  light  fall  upon  its 
strangely  sensitive  surface,  and  some  subtle  invis- 
ible change  takes  place.  It  is  then  merely  nec- 
essary to  plunge  the  plate  into  a  properly  pre- 
pared chemical  bath,  and  the  gradual  process  of 

81 


PHOTOGRAPHY    IN   ASTRONOMY 

developing  the  picture  begins.  Slowly,  very 
slowly,  the  colorless  surface  darkens  wherever 
light  has  touched  it.  Let  us  imagine  that  the 
exposure  has  been  made  with  an  ordinary  lens 
and  camera,  and  that  it  is  a  landscape  seeming  to 
grow  beneath  the  experimenter's  eyes.  At  first 
only  the  most  conspicuous  objects  make  their 
appearance.  But  gradually  the  process  extends, 
until  finally  every  tiny  detail  is  reproduced  with 
marvellous  fidelity  to  the  original.  The  photo- 
graphic plate,  when  developed  in  this  way,  is 
called  a  "  negative."  For  in  Nature  luminous 
points,  or  sources  of  light,  are  bright,  while  the 
developing  negative  turns  dark  wherever  light 
has  acted.  Thus  the  negative,  while  true  to  Nat- 
ure, reproduces  everything  in  a  reversed  way; 
bright  things  are  dark,  and  shadows  appear  light. 
For  ordinary  purposes,  therefore,  the  negative  has 
to  be  replaced  by  a  new  photograph  made  by  copy- 
ing it  again  photographically.  In  this  way  it  is 
again  reversed,  giving  us  a  picture  corresponding 
correctly  to  the  facts  as  seen.  Such  a  copy  from 
a  negative  is  what  is  ordinarily  called  a  photo- 
graph ;  technically,  it  is  known  as  a  "  positive." 

82 


PHOTOGRAPHY   IN  ASTRONOMY 

One  of  the  remarkable  things  about  the  sensi- 
tive plate  is  its  complete  indifference  to  the  dis- 
tance from  which  the  light  comes.  It  is  ready 
to  yield  obediently  to  the  ray  of  some  distant 
star  that  may  have  journeyed,  as  it  were,  from 
the  very  vanishing  point  of  space,  or  to  the 
bright  glow  of  an  electric  light  upon  the  photog- 
rapher's table.  This  quality  makes  its  use  es- 
pecially advantageous  in  astronomy,  since  we  can 
gain  knowledge  of  remote  stars  only  by  a  study 
of  the  light  they  send  us.  In  such  study  the 
photographic  plate  possesses  a  supreme  advan- 
tage over  the  human  eye.  If  the  conditions  of 
weather  and  atmosphere  are  favorable,  an  ob- 
server looking  through  an  ordinary  telescope 
will  see  nearly  as  much  at  the  first  glance  as  he 
will  ever  see.  Attentive  and  continued  study  will 
enable  him  to  fix  details  upon  his  memory,  and 
to  record  them  by  means  of  drawings  and  dia- 
grams. Occasional  moments  of  especially  un- 
disturbed atmospheric  conditions  will  allow  him 
to  glimpse  faint  objects  seldom  visible.  But  on 
the  whole,  telescopic  astronomers  add  little  to 
their  harvest  by  continued  husbandry  in  the 

83 


PHOTOGRAPHY   IN   ASTRONOMY 

same  field  of  stars.  Photography  is  different. 
The  effect  of  light  upon  the  sensitive  surface  of 
the  plate  is  strictly  cumulative.  If  a  given  star 
can  bring  about  a  certain  result  when  it  has  been 
allowed  to  act  upon  the  plate  for  one  minute, 
then  in  two  or  three  minutes  it  will  accomplish 
much  more.  Perhaps  a  single  minute's  exposure 
would  have  produced  a  mark  scarcely  perceptible 
upon  the  developed  negative.  In  that  case, 
three  or  four  minutes  would  give  us  a  perfectly 
well  defined  black  image  of  the  star. 

Thus,  by  lengthening  the  exposure  we  can 
make  the  fainter  stars  impress  themselves  upon 
the  plate.  If  their  light  is  not  able  to  produce 
the  desired  effect  in  minutes,  we  can  let  its  action 
accumulate  for  hours.  In  this  manner  it  be- 
comes possible  and  easy  to  photograph  objects 
so  faint  that  they  have  never  been  seen,  even 
with  our  most  powerful  telescopes.  This 
achievement  ranks  high  among  those  which 
make  astronomy  appeal  so  strongly  to  the  imag- 
ination. Scientific  men  are  not  given  to  fancies  ; 
nor  should  they  be.  But  the  first  long-exposure 
photograph  must  have  been  an  exciting  thing. 

84 


Star-Field  in  Constellation   Monoceros. 

Photographed  by  Barnard,  February  I,  1894. 
Exposure,  three  hours. 


PHOTOGRAPHY    IN   ASTRONOMY 

After  coming  from  the  observatory,  the  chemical 
development  was,  of  course,  made  in  a  dark 
room,  so  that  no  additional  light  might  harm  the 
plate  until  the  process  was  complete.  Carrying 
it  out  then  into  the  light,  that  early  experi- 
menter cannot  but  have  felt  a  thrill  of  triumph ; 
for  his  hand  held  a  true  picture  of  dim  stars  to 
the  eye  unlighted,  lifted  into  view  as  if  by  magic. 
Plates  have  been  thus  exposed  as  long  as 
twenty-five  hours,  and  the  manner  of  doing  it  is 
very  interesting.  Of  course,  it  is  impossible  to 
carry  on  the  work  continuously  for  so  long  a 
period,  since  the  beginning  of  daylight  would 
surely  ruin  the  photograph.  In  fact,  the  astron- 
omer must  stop  before  even  the  faintest  streak  of 
dawn  begins  to  redden  the  eastern  sky.  More- 
over, making  astronomical  negatives  requires  ex- 
cessively close  attention,  and  this  it  is  impossible 
to  give  continuously  during  more  than  a  few 
hours.  But  the  exposure  of  a  single  plate  can  be 
extended  over  several  nights  without  difficulty. 
It  is  merely  necessary  to  close  the  plate-holder 
with  a  "  light-tight  "  cover  when  the  first  night's 
work  is  finished.  To  begin  further  exposure  of 

85 


PHOTOGRAPHY   IN   ASTRONOMY 

the  same  plate  on  another  night,  we  simply  aim 
the  photographic  telescope  at  precisely  the  same 
point  of  the  sky  as  before.  The  light-tight 
plate-holder  being  again  opened,  the  exposure 
can  go  on  as  if  there  had  been  no  interruption. 

Astronomers  have  invented  a  most  ingenious 
device  for  making  sure  that  the  telescope's  aim 
can  be  brought  back  again  to  the  same  point  with 
great  exactness.  This  is  a  very  important  mat- 
ter ;  for  the  slightest  disturbance  of  the  plate  be- 
fore the  second  or  subsequent  portions  of  the  ex- 
posure would  ruin  everything.  Instead  of  a  very 
complete  single  picture,  we  should  have  two  partial 
ones  mixed  up  together  in  inextricable  confusion. 

To  prevent  this,  photographic  telescopes  are 
made  double,  not  altogether  unlike  an  opera-glass. 
One  of  the  tubes  is  arranged  for  photography 
proper,  while  the  other  is  fitted  with  lenses  suitable 
for  an  ordinary  visual  telescope.  The  two  tubes 
are  made  parallel.  Thus  the  astronomer,  by  look- 
ing through  the  visual  glass,  can  watch  objects  in 
the  heavens  even  while  they  are  being  photo- 
graphed. The  visual  half  of  the  instrument  is 
provided  with  a  pair  of  very  fine  cross-wires  mov- 

86 


PHOTOGRAPHY   IN   ASTRONOMY 

able  at  will  in  the  field  of  view.  These  can  be 
made  to  bisect  some  little  star  exactly,  before  be- 
ginning the  first  night's  work.  A  fterward,  every- 
thing about  the  instrument  having  been  left  un- 
changed, the  astronomer  can  always  assure  himself 
of  coming  back  to  precisely  the  same  point  of  the 
sky,  by  so  adjusting  the  instrument  that  the  same 
little  star  is  again  bisected. 

It  must  not  be  supposed,  however,  that  the 
entire  instrument  remains  unmoved,  even  during 
the  whole  of  a  single  night's  exposure.  For  in 
that  case,  the  apparent  motion  of  the  stars  as  they 
rise  or  set  in  the  sky  would  speedily  carry  them 
out  of  the  telescope's  field  of  view.  Consequent- 
ly, this  motion  has  to  be  counteracted  by  shift- 
ing the  telescope  so  as  to  follow  the  stars.  This 
can  be  accomplished  accurately  and  automatically 
by  means  of  clock-work  mechanism.  Such  con- 
trivances have  already  been  applied  in  the  past  to 
visual  telescopes,  because  even  then  they  facili- 
tated the  observer's  work.  They  save  him  the 
trouble  of  turning  his  instrument  every  few  min- 
utes, and  allow  him  to  give  his  undivided  atten- 
tion to  the  actual  business  of  observation. 

87 


PHOTOGRAPHY   IN   ASTRONOMY 

For  photographic  purposes  the  telescope  needs 
to  "  follow  "  the  stars  far  more  accurately  than  in 
the  older  kind  of  observing  with  the  eye.  Nor  is 
it  possible  to  make  a  clock  that  will  drive  the  in- 
strument satisfactorily  and  quite  automatically. 
But  by  means  of  the  second  or  visual  telescope, 
astronomers  can  always  ascertain  whether  the 
clock  is  working  correctly  at  any  given  moment. 
It  requires  only  a  glance  at  the  little  star  bisected 
by  the  cross-wires,  and,  if  there  has  been  the 
slightest  imperfection  in  the  following  by  clock- 
work, the  star  will  no  longer  be  cut  exactly  by 
the  wires. 

The  astronomer  can  at  once  correct  any  error 
by  putting  in  operation  a  very  ingenious  me- 
chanical device  sometimes  called  a  "mouse- 
control."  He  need  only  touch  an  electric  but- 
ton, and  a  signal  is  sent  into  the  clock-work. 
Instantly  there  is  a  shifting  of  the  mechanism. 
For  one  of  the  regular  driving  wheels  is  substi- 
tuted, temporarily,  another  having  an  extra  tooth. 
This  makes  the  clock  run  a  little  faster  so  long 
as  the  electric  current  passes.  In  a  similar  way, 
by  means  of  another  button,  the  clock  can  be 

88 


PHOTOGRAPHY   IN  ASTRONOMY 

made  to  run  slower  temporarily.  Thus  by 
watching  the  cross-wires  continuously,  and  ma- 
nipulating his  two  electric  buttons,  the  photo- 
graphic astronomer  can  compel  his  telescope  to 
follow  exactly  the  object  under  observation,  and 
he  can  make  certain  of  obtaining  a  perfect  neg- 
ative. 

These  long-exposure  plates  are  intended  espe- 
cially for  what  may  be  called  descriptive  astron- 
omy. With  them,  as  we  have  seen,  advantage  is 
taken  of  cumulative  light-effects  on  the  sensitive 
plate,  and  the  telescope's  light  -  gathering  and 
space  -  penetrating  powers  are  vastly  increased. 
We  are  enabled  to  carry  our  researches  far  be- 
yond the  confines  of  the  old  visible  universe. 
Extremely  faint  objects  can  be  recorded,  even 
down  to  their  minutest  details,  with  a  fidelity  un- 
known to  older  visual  methods.  But  at  present 
we  intend  to  consider  principally  applications 
of  photography  in  the  astronomy  of  measure- 
ment, rather  than  the  descriptive  branch  of  our 
subject.  Instead  of  describing  pictures  made 
simply  to  see  what  certain  objects  look  like  in 
the  sky,  we  shall  consider  negatives  intended  for 

89 


PHOTOGRAPHY  IN   ASTRONOMY 

precise  measurement,  with  all  that  the  word  pre- 
cision implies  in  celestial  science. 

Taking  up  first  the  photography  of  stars,  we 
must  begin  by  mentioning  the  work  of  Ruther- 
furd  at  New  York.  More  than  thirty  years  ago 
he  had  so  far  perfected  methods  of  stellar  pho- 
tography that  he  was  able  to  secure  excellent 
pictures  of  stars  as  faint  as  the  ninth  magnitude. 
In  those  days  the  modern  process  of  dry-plate 
photography  had  not  been  invented.  To-day, 
plates  exposed  in  the  photographic  telescope  are 
made  of  glass  covered  with  a  perfectly  dry  film 
of  sensitized  gelatine.  But  in  the  old  wet-plate 
process  the  sensitive  film  was  first  wetted  with  a 
chemical  solution ;  and  this  solution  could  not  be 
allowed  to  dry  during  the  exposure.  Conse- 
quently, Rutherfurd  was  limited  to  exposures  a 
few  minutes  in  length,  while  nowadays,  as  we 
have  said,  their  duration  can  be  prolonged  at  will. 

When  we  add  to  this  the  fact  that  the  old 
plates  were  far  less  sensitive  to  light  than  those 
now  available,  it  is  easy  to  see  what  were  the  diffi- 
culties in  the  way  of  photographing  faint  stars 
in  Rutherfurd's  time.  Nor  did  he  possess  the 

90 


PHOTOGRAPHY   IN   ASTRONOMY 

modern  ingenious  device  of  a  combined  visual 
and  photographic  instrument.  He  had  no  elec- 
tric controlling  apparatus.  In  fact,  the  younger 
generation  of  astronomers  can  form  no  adequate 
idea  of  the  patience  and  personal  skill  Ruther- 
furd  must  have  had  at  his  command.  For  he 
certainly  did  produce  negatives  that  are  but  little 
inferior  to  the  best  that  can  be  made  to-day. 
His  only  limitation  was  that  he  could  not 
obtain  images  of  stars  much  below  the  ninth 
magnitude. 

To  understand  just  what  is  meant  here  by  the 
ninth  magnitude,  it  is  necessary  to  go  back  in  im- 
agination to  the  time  of  Hipparchus,  the  father 
of  sidereal  astronomy.  (See  page  39.)  He 
adopted  the  convenient  plan  of  dividing  all  the 
stars  visible  to  the  naked  eye  (of  course,  he  had 
no  telescope)  into  six  classes,  according  to  their 
brilliancy.  The  faintest  visible  stars  were  put  in 
the  sixth  class,  and  all  the  others  were  assigned 
somewhat  arbitrarily  to  one  or  the  other  of  the 
brighter  classes. 

Modern  astronomers  have  devised  a  more  sci- 
entific system,  which  has  been  made  to  conform 

91 


PHOTOGRAPHY   IN   ASTRONOMY 

very  nearly  to  that  of  Hipparchus,  just  as  it  has 
come  down  to  us  through  the  ages.  We  have 
adopted  a  certain  arbitrary  degree  of  luminosity 
as  the  standard  "  first-magnitude";  compared  with 
sunlight,  this  may  be  represented  roughly  by  a 
fraction  of  which  the  numerator  is  i,  and  the  de- 
nominator about  eighty  thousand  millions.  The 
standard  second- magnitude  star  is  one  whose 
light,  compared  with  a  first-magnitude,  may  be 
represented  approximately  by  the  fraction  |-. 
The  third  magnitude,  in  turn,  may  be  compared 
with  the  second  by  the  same  fraction  f ;  and  so 
the  classification  is  extended  to  magnitudes  below 
those  visible  to  the  unaided  eye.  Each  magni- 
tude compares  with  the  one  above  it,  as  the  light 
of  two  candles  would  compare  with  the  light  of 
five. 

Rutherfurd  did  not  stop  with  mere  photo- 
graphs. He  realized  very  clearly  the  obvious 
truth  that  by  making  a  picture  of  the  sky  we 
simply  change  the  scene  of  our  operations. 
Upon  the  photograph  we  can  measure  that  which 
we  might  have  studied  directly  in  the  heavens ; 
but  so  long  as  they  remain  unmeasured,  celestial 

92 


PHOTOGRAPHY    IN   ASTRONOMY 

pictures  have  a  potential  value  only.  Locked 
within  them  may  lie  hidden  some  secret  of  our 
universe.  But  it  will  not  come  forth  unsought. 
Patient  effort  must  precede  discovery,  in  pho- 
tography, as  elsewhere  in  science.  There  is  no 
royal  road.  Rutherfurd  devised  an  elaborate 
measuring-machine  in  which  his  photographs 
could  be  examined  under  the  microscope  with 
the  most  minute  exactness.  With  this  machine 
he  measured  a  large  number  of  his  pictures  ;  and 
it  has  been  shown  quite  recently  that  the  results 
obtained  from  them  are  comparable  in  accuracy 
with  those  coming  from  the  most  highly  ac- 
credited methods  of  direct  eye-observation. 

And  photographs  are  far  superior  in  ease  of  ma- 
nipulation. Convenient  day-observing  under  the 
microscope  in  a  comfortable  astronomical  labora- 
tory is  substituted  for  all  the  discomforts  of  a 
midnight  vigil  under  the  stars.  The  work  of 
measurement  can  proceed  in  all  weathers,  whereas 
formerly  it  was  limited  strictly  to  perfectly  clear 
nights.  Lastly,  the  negatives  form  a  permanent 
record,  to  which  we  can  always  return  to  correct 
errors  or  re-examine  doubtful  points. 

93 


PHOTOGRAPHY   IN   ASTRONOMY 

Rutherford's  stellar  work  extended  down  to 
about  1877,  and  included  especially  parallax  de- 
terminations and  the  photography  of  star-clusters. 
Each  of  these  subjects  is  receiving  close  attention 
from  later  investigators,  and,  therefore,  merits 
brief  mention  here.  Stellar  parallax  is  in  one 
sense  but  another  name  for  stellar  distance.  Its 
measurement  has  been  one  of  the  important 
problems  of  astronomy  for  centuries,  ever  since 
men  recognized  that  the  Copernican  theory  of 
our  universe  requires  the  determination  of  stellar 
distances  for  its  complete  demonstration. 

If  the  earth  is  swinging  around  the  sun  once  a 
year  in  a  mighty  path  or  orbit,  there  must  be 
changes  of  its  position  in  space  comparable  in  size 
with  the  orbit  itself.  And  the  stars  ought  to  shift 
their  apparent  places  on  the  sky  to  correspond 
with  these  changes  in  the  terrestrial  observer's 
position.  The  phenomenon  is  analogous  to  what 
occurs  when  we  look  out  of  a  room,  first  through 
one  window,  and  then  through  another.  Any 
object  on  the  opposite  side  of  the  street  will  be 
seen  in  a  changed  direction,  on  account  of  the 
observer's  having  shifted  his  position  from  one 

94 


PHOTOGRAPHY   IN  ASTRONOMY 

window  to  the  other.  If  the  object  seemed  to 
be  due  north  when  seen  from  the  first  window, 
it  will,  perhaps,  appear  a  little  east  of  north  from 
the  other.  But  this  change  of  direction  will  be 
comparatively  small,  if  the  object  under  observa- 
tion is  very  far  away,  in  comparison  with  the  dis- 
tance between  the  two  windows. 

This  is  what  occurs  with  the  stars.  The  earth's 
orbit,  vast  as  it  is,  shrinks  into  almost  absolute 
insignificance  when  compared  with  the  profound 
distances  by  which  we  are  sundered  from  even 
the  nearest  fixed  stars.  Consequently,  the  shift- 
ing of  their  positions  is  also  very  small  —  so 
small  as  to  be  near  the  extreme  limit  separating 
that  which  is  measurable  from  that  which  is  be- 
yond human  ken. 

Photography  lends  itself  most  readily  to  a 
study  of  this  matter.  Suppose  a  certain  star  is 
suspected  of  "having  a  parallax."  In  other 
words,  we  have  reason  to  believe  it  near  enough 
to  admit  of  a  successful  measurement  of  distance. 
Perhaps  it  is  a  very  bright  star ;  and,  other 
things  being  equal,  it  is  probably  fair  to  assume 
that  brightness  signifies  nearness.  And  astrono- 

95 


PHOTOGRAPHY   IN   ASTRONOMY 

mers  have  certain  other  indications  of  proximity 
that  guide  them  in  the  selection  of  proper  objects 
for  investigation,  though  such  evidence,  of  course, 
never  takes  the  place  of  actual  measurement. 

The  star  under  examination  is  sure  to  have  near 
it  on  the  sky  a  number  of  stars  so  very  small 
that  we  may  safely  take  them  to  be  immeasurably 
far  away.  The  parallax  star  is  among  them, 
but  not  of  them.  We  see  it  projected  upon  the 
background  of  the  heavens,  though  it  may  in 
reality  be  quite  near  us,  astronomically  speaking. 
If  this  is  really  so,  and  the  star,  therefore,  subject 
to  the  slight  parallactic  shifting  already  men- 
tioned, we  can  detect  it  by  noting  the  suspected 
star's  position  among  the  surrounding  small 
stars.  For  these,  being  immeasurably  remote, 
will  remain  unchanged,  within  the  limits  of  our 
powers  of  observation,  and  thus  serve  as  points 
of  reference  for  marking  the  apparent  shifting  of 
the  brighter  star  we  are  actually  considering. 

We  have  merely  to  photograph  the  region  at 
various  seasons  of  the  year.  Careful  examina- 
tion of  the  photographs  under  the  microscope 

will  then  enable  us  to  measure  the  slightest  dis- 

96 


PHOTOGRAPHY    IN   ASTRONOMY 

placement  of  the  parallax  star.  From  these 
measures,  by  a  process  of  calculation,  astrono- 
mers can  then  obtain  the  star's  distance.  It  will 
not  become  known  in  miles ;  we  shall  only  ascer- 
tain how  many  times  the  distance  between  the 
earth  and  sun  would  have  to  be  laid  down  like  a 
measuring-rod,  in  order  to  cover  the  space  sepa- 
rating us  from  the  star :  and  the  subsequent 
evaluation  of  this  distance  "  earth  to  sun  "  in 
miles  is  another  important  problem  in  whose  so- 
lution photography  promises  to  be  most  useful. 

The  above  method  of  measuring  stellar  distance 
is,  of  course,  subject  to  whatever  slight  uncertainty 
arises  from  the  assumption  that  the  small  stars 
used  for  comparison  are  themselves  beyond  the 
possibility  of  parallactic  shifting.  But  astron- 
omy possesses  no  better  method.  Moreover, 
the  number  of  small  stars  used  in  this  way  is,  of 
course,  much  larger  in  photography  than  it  ever 
can  be  in  visual  work.  In  the  former  process, 
all  surrounding  stars  can  be  photographed  at 
once  ;  in  the  latter  each  star  must  be  measured 
separately,  and  daylight  soon  intervenes  to  im- 
pose a  limit  on  numbers.  Usually  only  two  can 

97 


PHOTOGRAPHY    IN   ASTRONOMY 

be  used  ;  so  that  here  photography  has  a  most 
important  advantage.  It  minimizes  the  chance 
of  our  parallax  being  rendered  erroneous,  by  the 
stars  of  comparison  not  being  really  infinitely 
remote.  This  might  happen,  perhaps,  in  the 
case  of  one  or  two ;  but  with  an  average  result 
from  a  large  number  we  know  it  to  be  practically 
impossible. 

Cluster  work  is  not  altogether  unlike  "  paral- 
lax hunting  "  in  its  preliminary  stage  of  securing 
the  photographic  observations.  The  object  is  to 
obtain  an  absolutely  faithful  picture  of  a  star 
group,  just  as  it  exists  in  the  sky.  We  have 
every  reason  to  suppose  that  a  very  large  num- 
ber of  stars  condensed  into  one  small  spot  upon 
the  heavens  means  something  more  than  chance 
aggregation.  The  Pleiades  group  (page  10)  con- 
tains thousands  of  massive  stars,  doubtless  held 
together  by  the  force  of  their  mutual  gravita- 
tional attraction.  If  this  be  true,  there  must  be 
complex  orbital  motion  in  the  cluster  ;  and,  as 
time  goes  on,  we  should  actually  see  the  sepa- 
rate components  change  their  relative  positions, 
as  it  were,  before  our  eyes.  The  details  of  such 

98 


PHOTOGRAPHY   IN   ASTRONOMY 

motion  upon  the  great  scale  of  cosmic  space  offer 
one  of  the  many  problems  that  make  astronomy 
the  grandest  of  human  sciences. 

We  have  said  that  time  must  pass  before  we 
can  see  these  things ;  there  may  be  centuries  of 
waiting.  But  one  way  exists  to  hurry  on  the 
perfection  of  our  knowledge ;  we  must  increase 
the  precision  of  observations.  Motions  that 
would  need  the  growth  of  centuries  to  become 
visible  to  the  older  astronomical  appliances, 
might  yield  in  a  few  decades  to  more  delicate 
observational  processes.  Here  photography  is 
most  promising.  Having  once  obtained  a  sur- 
passingly accurate  picture  of  a  star-cluster,  we 
can  subject  it  easily  to  precise  microscopic  meas- 
urement. The  same  operations  repeated  at  a 
later  date  will  enable  us  to  compare  the  two 
series  of  measures,  and  thus  ascertain  the  mo- 
tions that  may  have  occurred  in  the  interval. 
The  Rutherfurd  photographs  furnish  a  veritable 
mine  of  information  in  researches  of  this  kind ; 
for  they  antedate  all  other  celestial  photographs 
of  precision  by  at  least  a  quarter-century,  and 
bring  just  so  much  nearer  the  time  when  definite 

99 


PHOTOGRAPHY    IN   ASTRONOMY 

knowledge  shall  replace  information  based  on 
reasoning  from  probabilities. 

Rutherfurd's  methods  showed  the  advantages 
of  photography  as  applied  to  individual  star- 
clusters.  It  required  only  the  attention  of  some 
astronomer  disposing  of  large  observational  facili- 
ties, and  accustomed  to  operations  upon  a  great 
scale,  to  apply  similar  methods  throughout  the 
whole  heavens.  In  the  year  1882  a  bright 
comet  was  very  conspicuous  in  the  southern 
heavens.  It  was  extensively  observed  from  the 
southern  hemisphere,  and  especially  at  the  British 
Royal  Observatory  at  the  Cape  of  Good  Hope. 

Gill,  director  of  that  institution,  conceived  the 
idea  that  this  comet  might  be  bright  enough 
to  photograph.  At  that  time,  comet  photogra- 
phy had  been  attempted  but  little,  if  at  all,  and  it 
was  by  no  means  sure  that  the  experiment  would 
be  successful.  Nor  was  Gill  well  acquainted  with 
the  work  of  Rutherfurd  ;  for  the  best  results  of 
that  astronomer  had  lain  dormant  many  years. 
He  was  one  of  those  men  with  whom  personal 
modesty  amounts  to  a  fault.  Loath  to  put  him- 
self forward  in  any  way,  and  disliking  to  rush 


100 


PHOTOGRAPHY   IN   ASTRONOMY 

into  print,  Rutherfurd  had  given  but  little  pub- 
licity to  his  work.  This  peculiarity  has,  doubt- 
less, delayed  his  just  reputation  ;  but  he  will  lose 
nothing  in  the  end  from  a  brief  postponement. 
Gill  must,  however,  be  credited  with  more  pene- 
tration than  would  be  his  due  if  Rutherfurd  had 
made  it  possible  for  others  to  know  that  he  had 
anticipated  many  of  the  newer  ideas. 

However  this  may  be,  the  comet  was  photo- 
graphed with  the  help  of  a  local  portrait  photog- 
rapher named  Allis.  When  Gill  and  Allis  fast- 
ened a  simple  portrait  camera  belonging  to  the 
latter  upon  the  tube  of  one  of  the  Cape  tele- 
scopes, and  pointed  it  at  the  great  comet,  they 
little  thought  the  experiment  would  lead  to  one 
of  the  greatest  astronomical  works  ever  at- 
tempted by  men.  Yet  this  was  destined  to  oc- 
cur. The  negative  they  obtained  showed  an 
excellent  picture  of  the  comet ;  but  what  was 
more  important  for  the  future  of  sidereal  astron- 
omy, it  was  also  quite  thickly  dotted  with  little 
black  points  corresponding  to  stars.  The  extra- 
ordinary ease  with  which  the  whole  heavens 
could  be  thus  charted  photographically  was 

IOI 


PHOTOGRAPHY   IN  ASTRONOMY 

brought  home  to  Gill  as  never  before.  It  was 
this  comet  picture  that  interested  him  in  the  ap- 
plication of  photography  to  star-charting ;  and 
without  his  interest  the  now  famous  astro-photo- 
graphic catalogue  of  the  heavens  would  probably 
never  have  been  made. 

After  considerable  preliminary  correspondence, 
a  congress  of  astronomers  was  finally  called  to 
meet  at  Paris  in  1887.  Representatives  of  the 
principal  observatories  and  civilized  governments 
were  present.  They  decided  that  the  end  of  the 
nineteenth  century  should  see  the  making  of  a 
great  catalogue  of  all  the  stars  in  the  sky,  upon  a 
scale  of  completeness  and  precision  surpassing 
anything  previously  attempted.  It  is  impossible 
to  exaggerate  the  importance  of  such  a  work ; 
for  upon  our  star-catalogues  depends  ultimately 
the  entire  structure  of  astronomical  science. 

The  work  was  far  too  vast  for  the  powers  of 
any  observatory  alone.  Therefore,  the  whole 
sky,  from  pole  to  pole,  was  divided  into  eighteen 
belts  or  zones  of  approximately  equal  area ;  and 
each  of  these  was  assigned  to  a  single  observa- 
tory to  be  photographed.  A  series  of  telescopes 

102 


PHOTOGRAPHY   IN  ASTRONOMY 

was  specially  constructed,  so  that  every  part  of 
the  work  should  be  done  with  the  same  type  of 
instrument.  As  far  as  possible,  an  attempt  was 
made  to  secure  uniformity  of  methods,  and  par- 
ticularly a  uniform  scale  of  precision.  To  cover 
the  entire  sky  upon  the  plan  proposed  no  less 
than  44,108  negatives  are  required;  and  most 
of  these  have  now  been  finished.  The  further 
measurement  of  the  pictures  and  the  drawing  up 
of  a  vast  printed  star-catalogue  are  also  well  un- 
der way.  One  of  the  participating  observatories, 
that  at  Potsdam,  Germany,  has  published  the  first 
volume  of  its  part  of  the  catalogue.  It  is  esti- 
mated that  this  observatory  alone  will  require 
twenty  quarto  volumes  to  contain  merely  the 
final  results  of  its  work  on  the  catalogue.  Alto- 
gether not  less  than  two  million  stars  will  find  a 
place  in  this,  our  latest  directory  of  the  heavens. 

Such  wholesale  methods  of  attacking  problems 
of  observational  astronomy  are  particularly  char- 
acteristic of  photography.  The  great  catalogue 
is,  perhaps,  the  best  illustration  of  this  tendency  ; 
but  of  scarcely  smaller  interest,  though  less  im- 
portant in  reality,  is  the  photographic  method  of 

103 


PHOTOGRAPHY   IN   ASTRONOMY 

dealing  with  minor  planets.  We  have  already 
said  (page  63)  that  in  the  space  between  the  orbits 
of  Mars  and  Jupiter  several  hundred  small  bodies 
are  moving  around  the  sun  in  ordinary  planetary 
orbits.  These  bodies  are  called  asteroids,  or 
minor  planets.  The  visual  method  of  discover- 
ing unknown  members  of  this  group  was  pain- 
fully tedious ;  but  photography  has  changed 
matters  completely,  and  has  added  immensely 
to  our  knowledge  of  the  asteroids. 

Wolf,  of  Heidelberg,  first  made  use  of  the 
new  process  for  minor-planet  discovery.  His 
method  is  sufficiently  ingenious  to  deserve  brief 
mention  again.  A  photograph  of  a  suitable  re- 
gion of  the  sky  was  made  with  an  exposure  last- 
ing two  or  three  hours.  Throughout  all  this 
time  the  instrument  was  manipulated  so  as  to 
follow  the  motion  of  the  heavens  in  the  way  we 
have  already  explained,  so  that  each  star  would 
appear  on  the  negative  as  a  small,  round,  black 
dot. 

But  if  a  minor  planet  happened  to  be  in  the 
region  covered  by  the  plate,  its  photographic 
image  would  be  very  different.  For  the  orbital 

104 


PHOTOGRAPHY   IN   ASTRONOMY 

motion  of  the  planet  about  the  sun  would  make 
it  move  a  little  among  the  stars  even  in  the  two 
or  three  hours  during  which  the  plate  was  ex- 
posed. This  motion  would  be  faithfully  repro- 
duced in  the  picture,  so  that  the  planet  would 
appear  as  a  short  curved  line  rather  than  a  well- 
defined  dot  like  a  star.  Thus  the  presence  of 
such  a  line-image  infallibly  denotes  an  asteroid. 

Subsequent  calculations  are  necessary  to  ascer- 
tain whether  the  object  is  a  planet  already  known 
or  a  genuine  new  discovery.  Wolf,  and  others 
using  his  method  in  recent  years,  have  made  im- 
mense additions  to  our  catalogue  of  asteroids. 
Indeed,  the  matter  was  beginning  to  lose  inter- 
est on  account  of  the  frequency  and  sameness 
of  these  discoveries,  when  the  astronomical  world 
was  startled  by  the  finding  of  the  Planet  of  1898. 
(Page  58.) 

On  August  27,  1898,  Witt,  of  Berlin,  discov- 
ered the  small  body  that  bears  the  number 
"433  "  in  the  list  of  minor  planets,  and  has  re- 
ceived the  name  Eros.  Its  important  peculiar- 
ity consists  in  the  exceptional  position  of  the 
orbit.  While  all  the  other  asteroids  are  farther 

105 


PHOTOGRAPHY   IN   ASTRONOMY 

from  the  sun  than  Mars,  and  less  distant  than 
Jupiter,  Eros  can  pass  within  the  orbit  of  the 
former.  At  times,  therefore,  it  will  approach 
our  earth  more  closely  than  any  other  permanent 
member  of  the  solar  system,  excepting  our  own 
moon.  So  it  is,  in  a  sense,  our  nearest  neigh- 
bor ;  and  this  fact  alone  makes  it  the  most  inter- 
esting of  all  the  minor  planets.  The  nineteenth 
century  was  opened  by  Piazzi's  well-known  dis- 
covery of  the  first  of  these  bodies  (page  59) ;  it 
is,  therefore,  fitting  that  we  should  find  the  most 
important  one  at  its  close.  We  are  almost  cer- 
tain that  it  will  be  possible  to  make  use  of  Eros 
to  solve  with  unprecedented  accuracy  the  most 
important  problem  in  all  astronomy.  This  is  the 
determination  of  our  earth's  distance  from  the  sun. 
When  considering  stellar  parallax,  we  have  seen 
how  our  observations  enable  us  to  measure  some 
of  the  stars'  distances  in  terms  of  the  distance 
"  earth  to  sun  "  as  a  unit.  It  is,  indeed,  the  fun- 
damental unit  for  all  astronomical  measures,  and 
its  exact  evaluation  has  always  been  considered 
the  basal  problem  of  astronomy.  Astronomers 
know  it  as  the  problem  of  Solar  Parallax. 

106 


PHOTOGRAPHY   IN   ASTRONOMY 

We  shall  not  here  enter  into  the  somewhat 
intricate  details  of  this  subject,  however  interest- 
ing they  may  be.  The  problem  offers  diffi- 
culties somewhat  analogous  to  those  confronting 
a  surveyor  who  has  to  determine  the  distance  of 
some  inaccessible  terrestrial  point.  To  do  this, 
it  is  necessary  first  to  measure  a  "  base-line,"  as 
we  call  it.  Then  the  measurement  of  angles 
with  a  theodolite  will  make  it  possible  to  deduce 
the  required  distance  of  the  inaccessible  point  by 
a  process  of  calculation.  To  insure  accuracy, 
however,  as  every  surveyor  knows,  the  base-line 
must  be  made  long  enough  ;  and  this  is  precise- 
ly what  is  impossible  in  the  case  of  the  solar 
parallax. 

For  we  are  necessarily  limited  to  marking 
out  our  base-line  on  the  earth ;  and  the  entire 
planet  is  too  small  to  furnish  one  of  really  suffi- 
cient size.  The  best  we  can  do  is  to  use  the  dis- 
tance between  two  observatories  situated,  as  near 
as  may  be,  on  opposite  sides  of  the  earth.  But 
even  this  base  is  wofully  small.  However,  the 
smallness  loses  some  of  its  harmful  effect  if  we 
operate  upon  a  planet  that  is  comparatively  near 

107 


PHOTOGRAPHY    IN   ASTRONOMY 

us.  We  can  measure  such  a  planet's  distance 
more  accurately  than  any  other ;  and  this  being 
known,  the  solar  distance  can  be  computed  by 
the  aid  of  mathematical  considerations  based 
upon  Newton's  law  of  gravitation  and  observa- 
tional determinations  of  the  planetary  orbital 
elements. 

Photography  is  by  no  means  limited  to  inves- 
tigations in  the  older  departments  of  astronom- 
ical observation.  Its  powerful  arm  has  been 
stretched  out  to  grasp  as  well  the  newer  instru- 
ments of  spectroscopic  study.  Here  the  sensi- 
tive plate  has  been  substituted  for  the  human 
eye  with  even  greater  relative  advantage.  The 
accurate  microscopic  measurement  of  difficult 
lines  in  stellar  spectra  was  indeed  possible  by 
older  methods ;  but  photography  has  made  it 
comparatively  easy ;  and,  above  all,  has  ren- 
dered practicable  series  of  observations  extensive 
enough  in  numbers  to  furnish  statistical  informa- 
tion of  real  value.  Only  in  this  way  have  we 
been  able  to  determine  whether  the  stars,  in  their 
varied  and  unknown  orbits,  are  approaching  us 
or  moving  farther  away.  Even  the  speed  of  this 

108 


Solar  Corona.      Total  Eclipse. 
Photographed  by  Campbell,  January  22,    1898  ;  Jeur,   India. 


PHOTOGRAPHY   IN   ASTRONOMY 

approach  or  recession  has  become  measurable, 
and  has  been  evaluated  in  the  case  of  many  in- 
dividual stars.  (See  page  21.) 

The  subject  of  solar  physics  has  become  a  ver- 
itable department  of  astronomy  in  the  hands  of 
photographic  investigators.  Ingenious  spectro- 
photographic  methods  have  been  devised,  where- 
by we  have  secured  pictures  of  the  sun  from 
which  we  have  learned  much  that  must  have 
remained  forever  unknown  to  older  methods. 

Especially  useful  has  photography  proved  itself 
in  the  observation  of  total  solar  eclipses.  It  is 
only  when  the  sun's  bright  disk  is  completely 
obscured  by  the  interposed  moon  that  we  can  see 
the  faintly  luminous  structure  of  the  solar  co- 
rona, that  great  appendage  of  our  sun,  whose 
exact  nature  is  still  unexplained.  Only  during 
the  few  minutes  of  total  eclipse  in  each  century 
can  we  look  upon  it ;  and  keen  is  the  interest  of 
astronomers  when  those  few  minutes  occur.  But 
it  is  found  that  eye  observations  made  in  hur- 
ried excitement  have  comparatively  little  value. 
Half  a  dozen  persons  might  make  drawings  of 

the    corona   during    the   same    eclipse,  yet    they 

109 


PHOTOGRAPHY    IN   ASTRONOMY 

would  differ  so  much  from  one  another  as  to 
leave  the  true  .outline  very  much  in  doubt.  But 
with  photography  we  can  obtain  a  really  correct 
picture  whose  details  can  be  studied  and  dis- 
cussed subsequently  at  leisure. 

If  we  were  asked  to  sum  up  in  one  word  what 
photography  has  accomplished,  we  should  say 
that  observational  astronomy  has  been  revolu- 
tionized. There  is  to-day  scarcely  an  instru- 
ment of  precision  in  which  the  sensitive  plate 
has  not  been  substituted  for  the  human  eye ; 
scarcely  an  inquiry  possible  to  the  older  method 
which  cannot  now  be  undertaken  upon  a  grander 
scale.  Novel  investigations  formerly  not  even 
possible  are  now  entirely  practicable  by  photog- 
raphy ;  and  the  end  is  not  yet.  Valuable  as  are 
the  achievements  already  consummated,  photog- 
raphy is  richest  in  its  promise  for  the  future. 
Astronomy  has  been  called  the  "  perfect  sci- 
ence "  ;  it  is  safe  to  predict  that  the  next  gen- 
eration will  wonder  that  the  knowledge  we  have 
to-day  should  ever  have  received  so  proud  a 
title. 


no 


TIME    STANDARDS    OF    THE 
WORLD 

THE  question  is  often  asked,  "  What  is  the 
practical  use  of  astronomy  ?  "  We  know,  of 
course,  that  men  would  profit  greatly  from  a  study 
of  that  science,  even  if  it  could  not  be  turned  to 
any  immediate  bread-and-butter  use ;  for  astron- 
omy is  essentially  the  science  of  big  things,  and  it 
makes  men  bigger  to  fix  their  minds  on  problems 
that  deal  with  vast  distances  and  seemingly  end- 
less periods  of  time.  No  one  can  look  upon  the 
quietly  shining  stars  without  being  impressed  by 
the  thought  of  how  they  burned — then  as  now — 
before  he  himself  was  born,  and  so  shall  continue 
after  he  has  passed  away — aye,  even  after  his  lat- 
est descendants  shall  have  vanished  from  the 
earth.  Of  all  the  sciences,  astronomy  is  at  once 
the  most  beautiful  poetically,  and  yet  the  one 
offering  the  grandest  and  most  difficult  problems 
to  the  intellect.  A  study  of  these  problems  has 


in 


TIME    STANDARDS    OF   THE   WORLD 

ing  to  wind  his  watch  at  the  accustomed  hour. 
The  next  morning  he  finds  it  run  down.  It 
must  be  re-set.  Most  people  simply  go  to  the 
nearest  clock,  or  ask  some  friend  for  the  time,  so 
as  to  start  the  watch  correctly.  More  careful  per- 
sons, perhaps,  visit  the  jeweller's  and  take  the 
time  from  his  "  regulator."  But  the  regulator 
itself  needs  to  be  regulated.  After  all,  it  is  noth- 
ing more  than  any  other  clock,  except  that  greater 
care  has  been  taken  in  the  mechanical  construction 
and  arrangement  of  its  various  parts.  Yet  it  is 
but  a  machine  built  by  human  hands,  and,  like 
all  human  works,  it  is  necessarily  imperfect.  No 
matter  how  well  it  has  been  constructed,  it  will 
not  run  with  perfectly  rigid  accuracy.  Every  day 
there  will  be  a  variation  from  the  true  time  by  a 
small  amount,  and  in  the  course  of  days  or  weeks 
the  accumulation  of  these  successive  small  amounts 
will  lead  to  a  total  of  quite  appreciable  size. 

Just  as  the  ordinary  citizen  looks  to  the  jewel- 
ler's regulator  to  correct  his  watch,  so  the  jeweller 
applies  to  the  astronomer  for  the  correction  of  his 
regulator.  Ever  since  the  dawn  of  astronomy,  in 

the  earliest  ages  of  which  we  have  any  record,  the 

114 


TIME    STANDARDS    OF   THE   WORLD 

principal  duty  of  the  astronomer  has  been  the  fur- 
nishing of  accurate  time  to  the  people.  We  shall 
not  here  enter  into  a  detailed  account,  however  in- 
teresting it  would  be,  of  the  gradual  development 
by  which  the  very  perfect  system  at  present  in 
use  has  been  reached  ;  but  shall  content  ourselves 
with  a  description  of  the  methods  now  employed 
in  nearly  all  the  civilized  countries  of  the  world. 

In  the  first  place,  every  observatory  is,  of  course, 
provided  with  what  is  known  as  an  astronomical 
clock.  This  instrument,  from  the  astronomer's 
point  of  view,  is  something  very  different  from 
the  ordinary  popular  idea.  To  the  average  per- 
son an  astronomical  clock  is  a  complicated  and 
elaborate  affair,  giving  the  date,  day  of  the  week, 
phases  of  the  moon,  and  other  miscellaneous  in- 
formation. But  in  reality  the  astronomer  wants 
none  of  these  things.  His  one  and  only  require- 
ment is  that  the  clock  shall  keep  as  near  uniform 
time  as  may  be  possible  to  a  machine  constructed 
by  human  hands.  No  expense  is  spared  in  mak- 
ing the  standard  clock  for  an  observatory.  Real 
artists  in  mechanical  construction — men  who  have 
attained  a  world-wide  celebrity  for  delicate  skill 

"5 


TIME    STANDARDS    OF   THE   WORLD 

in  fashioning  the  parts  of  a  clock — such  are  the 
astronomer's  clock-makers. 

To  increase  precision  of  motion  in  the  train  of 
wheels,  it  is  necessary  that  the  mechanism  be  as 
simple  as  possible.  For  this  reason  all  complica- 
tions of  date,  etc.,  are  left  out.  We  have  even 
abandoned  the  usual  convenient  plan  of  having 
the  hour  and  minute  hands  mounted  at  the  same 
centre ;  for  this  kind  of  mounting  makes  neces- 
sary a  slightly  more  intricate  form  of  wheelwork. 
The  astronomer's  clock  usually  has  the  centres 
of  the  second  hand,  minute  hand,  and  hour  hand 
in  a  straight  line,  and  equally  distant  from  each 
other.  Each  hand  has  its  own  dial ;  all  drawn, 
of  course,  upon  the  same  clock-face. 

Even  after  such  a  clock  has  been  made  as  ac- 
curately as  possible,  it  will,  nevertheless,  not  give 
the  very  best  performance  unless  it  is  taken  care 
of  properly.  It  is  necessary  to  mount  it  very 
firmly  indeed.  It  should  not  be  fastened  to  an 
ordinary  wall,  but  a  strong  pier  of  masonry  or 
brick  must  be  built  for  it  on  a  very  solid  founda- 
tion. Moreover,  this  pier  is  best  placed  under- 
ground in  a  cellar,  so  that  the  temperature  of  the 

116 


TIME    STANDARDS    OF   THE   WORLD 

clock  can  be  kept  nearly  uniform  all  the  year 
round ;  for  we  find  that  clocks  do  not  run  quite 
the  same  in  hot  weather  as  they  do  in  cold. 
Makers  have,  indeed,  tried  to  guard  against  this 
effect  of  temperature,  by  ingenious  mechanical 
contrivances.  But  these  are  never  quite  perfect 
in  their  action,  and  it  is  best  not  to  test  them  too 
severely  by  exposing  the  clock  to  sharp  changes 
of  heat  and  cold. 

Another  thing  affecting  the  going  of  fine 
clocks,  strange  as  it  may  seem,  is  the  variation  of 
barometric  pressure.  There  is  a  slight  but  no- 
ticeable difference  in  their  running  when  the 
barometer  is  high  and  when  it  is  low.  To  pre- 
vent this,  some  of  our  best  clocks  have  been  en- 
closed in  air-tight  cases,  so  that  outside  barometric 
changes  may  not  be  felt  in  the  least  by  the  clock 
itself. 

But  even  after  all  this  has  been  accomplished, 
and  the  astronomer  is  in  possession  of  a  clock  that 
may  be  called  a  masterpiece  of  mechanical  con- 
struction, he  is  not  any  better  off  than  was  the 
jeweller  with  his  regulator.  After  all,  even  the 

astronomical  clock  needs  to  be  set,  and  its  error 

117 


TIME    STANDARDS    OF   THE   WORLD 

must  be  determined  from  time  to  time.  A 
final  appeal  must  then  be  had  to  astronomical 
observations.  The  clock  must  be  set  by  the 
stars  and  sun.  For  this  purpose  the  astronomer 
uses  an  instrument  called  a  "  transit."  This  is 
simply  a  telescope  of  moderate  size,  possibly  five 
or  six  feet  long,  and  firmly  attached  to  an  axis  at 
right  angles  to  the  tube  of  the  telescope. 

This  axis  is  supported  horizontally  in  such  a 
way  that  it  points  as  nearly  as  may  be  exactly 
east  and  west.  The  telescope  itself  being  square 
with  the  axis,  always  points  in  a  north-and-south 
direction.  It  is  possible  to  rotate  the  telescope 
about  its  axis  so  as  to  reach  all  parts  of  the  sky 
that  are  directly  north  or  south  of  the  observa- 
tory. In  the  field  of  view  of  the  telescope  cer- 
tain very  fine  threads  are  mounted  so  as  to  form 
a  little  cross.  As  the  telescope  is  rotated  this 
cross  traces  out,  as  it  were,  a  great  circle  on  the 
sky ;  and  this  great  circle  is  called  the  astronomi- 
cal meridian. 

Now  we  are  in  possession  of  certain  star-tables, 
computed  from  the  combined  observations  of 
astronomers  in  the  last  150  years.  These  tables 

us 


TIME    STANDARDS    OF  THE  WORLD 

tell  us  the  exact  moment  of  time  when  any  star 
is  on  the  meridian.  To  discover,  therefore, 
whether  our  clock  is  right  on  any  given  night,  it 
is  merely  necessary  to  watch  a  star  with  the  tele- 
scope, and  note  the  exact  instant  by  the  clock 
when  it  reaches  the  little  cross  in  the  field  of 
view.  Knowing  from  the  astronomical  tables  the 
time  when  the  star  ought  to  have  been  on  the 
meridian,  and  having  observed  the  clock  time 
when  it  is  actually  there,  the  difference  is,  of 
course,  the  error  of  the  clock.  The  result  can 
be  checked  by  observations  of  other  stars,  and 
the  slight  personal  errors  of  observation  can  be 
rendered  harmless  by  taking  the  mean  from  sev- 
eral stars.  By  an  hour's  work  on  a  fine  night  it 
is  possible  to  fix  the  clock  error  quite  easily 
within  the  one-twentieth  part  of  a  second. 

We  have  not  space  to  enter  into  the  interest- 
ing details  of  the  methods  by  which  the  astro- 
nomical transit  is  accurately  set  in  the  right 
position,  and  how  any  slight  residual  error  in 
its  setting  can  be  eliminated  from  our  results  by 
certain  processes  of  computation.  It  must  suffice 
to  say  that  practically  all  time  determinations  in 

119 


TIME    STANDARDS    OF   THE   WORLD 

the  observatory  depend  substantially  upon  the 
procedure  outlined  above. 

The  observatory  clock  having  been  once  set 
right  by  observations  of  the  sky,  its  error  can  be 
re-determined  every  few  days  quite  easily.  Thus 
even  the  small  irregularities  of  its  nearly  perfect 
mechanism  can  be  prevented  from  accumulating 
until  they  might  reach  a  harmful  magnitude. 
But  we  obtain  in  this  way  only  a  correct  standard 
of  time  within  the  observatory  itself.  How  can 
this  be  made  available  for  the  general  public  ? 
The  problem  is  quite  simple  with  the  aid  of  the 
electric  telegraph.  We  shall  give  a  brief  account 
of  the  methods  now  in  use  in  New  York  City, 
and  these  may  be  taken  as  essentially  representa- 
tive of  those  employed  elsewhere. 

Every  day,  at  noon  precisely,  an  electric  signal 
is  sent  out  by  the  United  States  Naval  Observa- 
tory in  Washington.  The  signal  is  regulated  by 
the  standard  clock  of  the  observatory,  of  course 
taking  account  of  star  observations  made  on  the 
next  preceding  fine  night.  This  signal  is  re- 
ceived in  the  central  New  York  office  of  the  tele- 
graph company,  where  it  is  used  to  keep  correct 


TIME    STANDARDS    OF   THE   WORLD 

a  very  fine  clock,  which  may  be  called  the  time 
standard  of  the  telegraph  company.  This  clock, 
in  turn,  has  automatic  electric  connections,  by 
means  of  which  it  is  made  to  send  out  signals 
over  what  are  called  "  time  wires "  that  go  all 
over  the  city.  Jewellers,  and  others  who  desire 
correct  time,  can  arrange  to  have  a  small  electric 
sounder  in  their  offices  connected  with  the  time 
wires.  Thus  the  ticks  of  the  telegraph  com- 
pany's standard  clock  are  repeated  automatically 
in  the  jeweller's  shop,  and  used  for  controlling 
the  exactness  of  his  regulator.  This,  in  brief,  is 
the  method  by  which  the  astronomer's  careful 
determination  of  correct  time  is  transferred  and 
distributed  to  the  people  at  large. 

Having  thus  outlined  the  manner  of  obtaining 
and  distributing  correct  time,  we  shall  now  consider 
the  question  of  time  differences  between  different 
places  on  the  earth.  This  is  a  matter  which  many 
persons  find  most  perplexing,  and  yet  it  is  essential- 
ly quite  simple  in  principle.  Travellers,  of  course, 
are  well  acquainted  with  the  fact  that  their  watches 
often  need  to  be  reset  when  they  arrive  at  their 
destination.  Yet  few  ever  stop  to  ask  the  cause. 

121 


TIME    STANDARDS    OF   THE   WORLD 

Let  us  consider  for  a  moment  our  method  of 
measuring  time.  We  go  by  the  sun.  If  we  leave 
out  of  account  some  small  irregularities  of  the  sun's 
motion  that  are  of  no  consequence  for  our  present 
purpose,  we  may  lay  down  this  fundamental  prin- 
ciple :  When  the  sun  reaches  its  highest  position 
in  the  sky  it  is  twelve  o'clock  or  noon. 

The  sun,  as  everyone  knows,  rises  each  morn- 
ing in  the  east,  slowly  goes  up  higher  and  higher 
in  the  sky,  and  at  last  begins  to  descend  again 
toward  the  west.  But  it  is  clear  that  as  the  sun 
travels  from  east  to  west,  it  must  pass  over  the 
eastern  one  of  any  two  cities  sooner  than  the 
western  one.  When  it  reaches  its  greatest  height 
over  a  western  city  it  has,  therefore,  already  passed 
its  greatest  height  over  an  eastern  one.  In  other 
words,  when  it  is  noon,  or  twelve  o'clock,  in  the 
western  city,  it  is  already  after  noon  in  the  "eastern 
city.  This  is  the  simple  and  evident  cause  of 
time  differences  in  different  parts  of  the  country. 
Of  any  two  places  the  eastern  one  always  has  later 
time  than  the  western.  When  we  consider  the 
matter  in  this  way  there  is  not  the  slightest  diffi- 
culty in  understanding  how  time  differences  arise. 


122 


TIME    STANDARDS    OF  THE   WORLD 

They  will,  of  course,  be  greatest  for  places  that 
are  very  far  apart  in  an  east-and-west  direction. 
And  this  brings  us  again  to  the  subject  of  longi- 
tude, which,  as  we  have  already  said,  plays  an 
important  part  in  all  questions  relating  to  time ; 
for  longitude  is  used  to  measure  the  distance  in 
an  east-and-west  direction  between  different  parts 
of  the  earth. 

If  we  consider  the  earth  as  a  large  ball  we  can 
imagine  a  series  of  great  circles  drawn  on  its  surface 
and  passing  directly  from  the  North  Pole  to  the 
South  Pole.  Such  a  circle  could  be  drawn  through 
any  point  on  the  earth.  If  we  imagine  a  pair  of 
them  drawn  through  two  cities,  such  as  New  York 
and  London,  the  longitude  difference  of  these  two 
cities  is  defined  as  the  angle  at  the  North  Pole  be- 
tween the  two  great  circles  in  question.  The  size 
of  this  angle  can  be  expressed  in  degrees.  If  we 
then  wish  to  know  the  difference  in  time  between 
New  York  and  London  in  hours,  we  need  only 
divide  their  longitude  difference  in  degrees  by  the 
number  15.  In  this  simple  way  we  can  get  the 
time  difference  of  any  two  places.  We  merely 

measure  the  longitude  difference  on  a  map,  and 

123 


TIME    STANDARDS    OF   THE   WORLD 

then  divide  by  15  to  get  the  time  difference. 
These  time  differences  can  sometimes  become 
quite  large.  Indeed,  for  two  places  differing  180 
degrees  in  longitude,  the  time  difference  will  evi- 
dently be  no  less  than  twelve  hours. 

Most  civilized  nations  have  agreed  informally 
to  adopt  some  one  city  as  the  fundamental  point 
from  which  all  longitudes  are  to  be  counted.  Up 
to  the  present  we  have  considered  only  longitude 
differences ;  but  when  we  speak  of  the  longitude 
of  a  city  we  mean  its  longitude  difference  from 
the  place  chosen  by  common  consent  as  the  ori- 
gin for  measuring  longitudes.  The  town  almost 
universally  used  for  this  purpose  is  Greenwich, 
near  London,  England.  Here  is  situated  the 
British  Royal  Observatory,  one  of  the  oldest  and 
most  important  institutions  of  its  kind  in  the 
world.  The  great  longitude  circle  passing  through 
the  centre  of  the  astronomical  transit  at  the  Green- 
wich observatory  is  the  fundamental  longitude 
circle  of  the  earth.  The  longitude  of  any  other 
town  is  then  simply  the  angle  at  the  pole  between 
the  longitude  circle  through  that  town  and  the 

fundamental  Greenwich  one  here  described. 

124 


TIME    STANDARDS    OF   THE   WORLD 

Longitudes  are  counted  both  eastward  and 
westward  from  Greenwich.  Thus  New  York  is 
in  74  degrees  west  longitude,  while  Berlin  is  in 
14  degrees  east  longitude.  This  has  led  to  a 
rather  curious  state  of  affairs  in  those  parts  of  the 
earth  the  longitudes  of  which  are  nearly  180 
degrees  east  or  west.  There  are  a  number  of 
islands  in  that  part  of  the  world,  and  if  we  imagine 
for  a  moment  one  whose  longitude  is  just  180  de- 
grees, we  shall  have  the  following  remarkable  re- 
sult as  to  its  time  difference  from  Greenwich. 

We  have  seen  that  of  any  two  places  the  eastern 
always  has  the  later  time.  Now,  since  our  imag- 
inary island  is  exactly  180  degrees  from  Green- 
wich, we  can  consider  it  as  being  either  1 80  de- 
grees east  or  180  degrees  west.  But  if  we  call  it 
1 80  degrees  east,  its  time  will  be  twelve  hours 
later  than  Greenwich,  and  if  we  call  it  180  degrees 
west,  its  time  will  be  twelve  hours  earlier  than 
Greenwich.  Evidently  there  will  be  a  difference 
of  just  twenty-four  hours,  or  one  whole  day,  be- 
tween these  two  possible  ways  of  reckoning  its 
time.  This  circumstance  has  actually  led  to  con- 
siderable confusion  in  some  of  the  islands  of  the 

125 


TIME    STANDARDS    OF  THE   WORLD 

Pacific  Ocean.  The  navigators  who  discovered 
the  various  islands  naturally  gave  them  the  date 
which  they  brought  from  Europe.  And  as  some 
of  these  navigators  sailed  eastward,  around  the 
Cape  of  Good  Hope,  and  others  westward,  around 
Cape  Horn,  the  dates  they  gave  to  the  several 
islands  differed  by  just  one  day. 

The  state  of  affairs  at  the  present  time  has 
been  adjusted  by  a  sort  of  informal  agreement. 
An  arbitrary  line  has  been  drawn  on  the  map 
near  the  iSoth  longitude  circle,  and  it  has  been 
decided  that  the  islands  on  the  east  side  of  this 
line  shall  count  their  longitudes  west  from  Green- 
wich, and  those  west  of  the  line  shall  count  lon- 
gitude east  from  Greenwich.  Thus  Samoa  is 
nearly  180  degrees  wesvt  of  Greenwich,  while  the 
Fiji  Islands  are  nearly  180  degrees  east.  Yet  the 
islands  are  very  near  each  other,  though  the  ar- 
bitrary line  passes  between  them.  As  a  result, 
when  it  is  Sunday  in  Samoa  it  is  Monday  in  the 
Fiji  Islands.  The  arbitrary  line  described  here 
is  sometimes  called  the  International  Date-Line. 

It   does    not    pass    very    near    the    Philippine 

Islands,  which  are  situated  in  about  120  degrees 

126 


TIME    STANDARDS    OF   THE   WORLD 

east  longitude,  and,  therefore,  use  a  time  about 
eight  hours  later  than  Greenwich.  New  York, 
being  about  74  degrees  west  of  Greenwich,  is 
about  five  hours  earlier  in  time.  Consequently, 
as  we  may  remark  in  passing,  Philippine  time  is 
about  thirteen  hours  later  than  New  York  time. 
Thus,  five  o'clock,  Sunday  morning,  May  ist, 
in  Manila,  would  correspond  to  four  o'clock, 
Saturday  afternoon,  April  3Oth,  in  New  York. 

There  is  another  kind  of  time  which  we  shall  ex- 
plain briefly — the  so-called  "  standard,"  or  railroad 
time,  which  came  into  general  use  in  the  United 
States  some  few  years  ago,  and  has  since  been 
generally  adopted  throughout  the  world.  It  re- 
quires but  a  few  moments'  consideration  to  see 
that  the  accidental  situation  of  the  different  large 
cities  in  any  country  will  cause  their  local  times 
to  differ  by  odd  numbers  of  hours,  minutes,  and 
seconds.  Thus  a  great  deal  of  inconvenience 
has  been  caused  in  the  past.  For  instance,  a 
train  might  leave  New  York  at  a  certain  hour  by 
New  York  time.  It  would  then  arrive  in  Buf- 
falo some  hours  later  by  New  York  time.  But 

it  would  leave  Buffalo  by  Buffalo  time,  which  is 

127 


TIME    STANDARDS    OF    THE   WORLD 

quite  different.  Thus  there  would  be  a  sort  ot 
jump  in  the  time-table  at  Buffalo,  and  it  would 
be  a  jump  of  an  odd  number  of  minutes. 

It  would  be  different  in  different  cities,  and  very 
hard  to  remember.  Indeed,  as  each  railway  usu- 
ally ran  its  trains  by  the  time  used  in  the  princi- 
pal city  along  its  line,  it  might  happen  that  three 
or  four  different  railroad  times  would  be  used  in 
a  single  city  where  several  roads  met.  This  has 
all  been  avoided  by  introducing  the  standard 
time  system.  According  to  this  the  whole  coun- 
try is  divided  into  a  series  of  time  zones,  fifteen 
degrees  wide,  and  so  arranged  that  the  middle 
line  of  each  zone  falls  at  a  point  whose  longitude 
from  Greenwich  is  60,  75,  90,  105,  or  120  de- 
grees. The  times  at  these  middle  lines  are, 
therefore,  earlier  than  Greenwich  time  by  an 
even  number  of  hours.  Thus,  for  instance,  the 
75~degree  line  is  just  five  even  hours  earlier  than 
Greenwich  time.  All  cities  simply  use  the  time 
of  the  nearest  one  of  these  special  lines. 

This  does  not  result  in  doing  away  with  time 
differences  altogether — that  would,  of  course,  be 
impossible  in  the  nature  of  things — but  for  the 

128 


TIME    STANDARDS    OF   THE   WORLD 

complicated  odd  differences  in  hours  and  minutes, 
we  have  substituted  the  infinitely  simpler  series 
of  differences  in  even  hours.  The  traveller  from 
Chicago  to  New  York  can  reset  his  watch  by 
putting  it  just  one  hour  later  on  his  arrival — the 
minute  hand  is  kept  unchanged,  and  no  New 
York  timepiece  need  be  consulted  to  set  the 
watch  right  on  arriving.  There  can  be  no  doubt 
that  this  standard-time  system  must  be  consid- 
ered one  of  the  most  important  contributions  of 
astronomical  science  to  the  convenience  of  man. 

Its  value  has  received  the  widest  recognition, 
and  its  use  has  now  extended  to  almost  all  civil- 
ized countries — France  is  the  only  nation  of  im- 
portance still  remaining  outside  the  time-zone 
system.  In  the  following  table  we  give  the 
standard  time  of  the  various  parts  of  the  earth  as 
compared  with  Greenwich,  together  with  the  date 
of  adopting  the  new  time  system.  It  will  be 
noticed  that  in  certain  cases  even  half-hours  have 
been  employed  to  separate  the  time-zones,  in- 
stead of  even  hours  as  used  in  the  United  States. 


129 


TIME    STANDARDS    OF   THE  WORLD 


TABLE    OF   THE   WORLD'S    TIME   STANDARDS 


When  it  is  Noon 
at  Greenwich 
it  is 

In 

Date  of  Adopting 
Standard  Time 
System. 

Noon 

Great  Britain. 

Belgium. 

May,  1892. 

Holland. 

May,  1892. 

Spain. 

January,  1901. 

I   P.M. 

Germany. 

April,  1893. 

Italy. 

November,  i  893. 

Denmark. 

January,  1894. 

Switzerland. 

June,  1894. 

Norway. 

January,  1895. 

Austria  (railways). 

1.30  P.M. 

Cape  Colony. 

1892. 

Orange  River  Colony. 

1892. 

Transvaal. 

1  1892. 

2  P.M. 

Natal. 

September,  1895. 

Turkey  (railways). 

Egypt. 

October,  1900. 

8  P.M. 

West  Australia. 

February,  1895. 

9  P.M. 

Japan. 

1896. 

9.30  P.M. 

South  Australia. 

May,  1899. 

IO  P.M. 

Victoria. 

February,  1895. 

New  South  Wales. 

February,  1895. 

Queensland. 

February,  1895. 

II    P.M. 

New  Zealand. 

In  the  United  States  and  Canada  it  is 

4  A.M.  by  Pacific   Time  when  it  is  Noon  at  Greenwich. 


5   A.M. 

'   Mountain  " 

6  A.M. 

"  Central      " 

<t       tt 

tt      tt          ft 

7  A.M. 
8   A.M. 

"   Colonial     " 

tt       tt 

n     tt          tt 

130 


MOTIONS  OF  THE  EARTH'S  POLE 

STUDENTS  of  geology  have  been  puzzled  for 
many  years  by  traces  remaining  from  the  period 
when  a  large  part  of  the  earth  was  covered  with  a 
heavy  cap  of  ice.  These  shreds  of  evidence  all 
seem  to  point  to  the  conclusion  that  the  centre  of 
the  ice-covered  region  was  quite  far  away  from 
the  present  position  of  the  north  pole  of  the  earth. 
If  we  are  to  regard  the  pole  as  very  near  the 
point  of  greatest  cold,  it  becomes  a  matter  of 
much  interest  to  examine  whether  the  pole  has 
always  occupied  its  present  position,  or  whether 
it  has  been  subject  to  slow  changes  of  place  upon 
the  earth's  surface.  Therefore,  the  geologists 
have  appealed  to  astronomers  to  discover  whether 
they  are  in  possession  of  any  observational  evi- 
dence tending  to  show  that  the  pole  is  in  motion. 

Now  we  may  say  at  once  that  astronomical  re- 
search has  not  as  yet  revealed  the  evidence  thus 
expected.  Astronomy  has  been  unable  to  come 

131 


MOTIONS   OF   THE   EARTH'S   POLE 

to  the  rescue  of  geological  theory.  From  about 
the  year  17 50,  which  saw  the  beginning  of  precise 
observation  in  the  modern  sense,  down  to  very 
recent  times,  astronomers  were  compelled  to  deny 
the  possibility  of  any  appreciable  motion  of  the 
pole.  Observational  processes,  it  is  true,  fur- 
nished slightly  divergent  pole  positions  from  time 
to  time.  Yet  these  discrepancies  were  always  so 
minute  as  to  be  indistinguishable  from  those  slight 
personal  errors  that  are  ever  inseparable  from  re- 
sults obtained  by  the  fallible  human  eye. 

But  in  the  last  few  years  improved  methods  of 
observation,  coupled  with  extreme  diligence  in 
their  application  by  astronomers  generally,  have 
brought  to  light  a  certain  small  motion  of  the 
pole  which  had  never  before  been  demonstrated 
in  a  reliable  way.  This  motion,  it  is  true,  is  not 
of  the  character  demanded  by  geological  theory , 
for  the  geologists  had  been  led  to  expect  a  motion 
which  would  be  continuous  in  the  same  direction, 
no  matter  how  slow  might  be  its  annual  amount ; 
for  the  vast  extent  of  geologic  time  would  give 
even  the  slowest  of  motions  an  opportunity  to 
produce  large  effects,  provided  its  results  could 

132 


MOTIONS   OF   THE   EARTH'S   POLE 

be  continuously  cumulative.  Given  time  enough, 
and  the  pole  might  move  anywhere  on  the  earth, 
no  matter  how  slow  might  be  its  tortoise  speed. 

But  the  small  motion  we  have  discovered  is 
neither  cumulative  nor  continuous  in  one  direc- 
tion. It  is  what  we  call  a  periodic  motion,  the 
pole  swinging  now  to  one  side,  and  now  to  the 
other,  of  its  mean  or  average  position.  Thus 
this  new  discovery  cannot  be  said  to  unravel  the 
mysterious  puzzle  of  the  geologists.  Yet  it  is 
not  without  the  keenest  interest,  even  from  their 
point  of  view;  for  the  proof  of  any  form  of 
motion  in  a  pole  previously  supposed  to  be  abso- 
lutely at  rest  may  mean  everything.  No  man 
can  say  what  results  will  be  revealed  by  the  fur- 
ther observations  now  being  continued  with  great 
diligence. 

In  the  first  place,  it  is  important  to  explain  that 
any  such  motions  as  we  have  under  considera- 
tion will  show  themselves  to  ordinary  observa- 
tional processes  principally  in  the  form  of  changes 
of  terrestrial  latitudes.  Let  us  imagine  a  pair  of 
straight  lines  passing  through  the  centre  of  the 
earth  and  terminating,  one  at  the  observer's  sta- 

133 


MOTIONS   OF  THE  EARTH'S   POLE 

tion  on  the  earth's  surface,  and  the  other  at  that 
point  of  the  equator  which  is  nearest  the  observer. 
Then,  according  to  the  ordinary  definition  of  lat- 
itude, the  angle  between  these  two  imaginary  lines 
is  called  the  latitude  of  the  point  of  observation. 
Now  we  know,  of  course,  that  the  equator  is 
everywhere  just  90  degrees  from  the  pole.  Con- 
sequently, if  the  pole  is  subject  to  any  motion  at 
all,  the  equator  must  also  partake  of  the  motion. 

Thus  the  angle  between  our  two  imaginary  lines 
will  be  affected  directly  by  polar  movement,  and 
the  latitude  obtained  by  astronomical  observation 
will  be  subject  to  quite  similar  changes.  To  clear 
up  the  whole  question,  so  far  as  this  can  be  done 
by  the  gathering  of  observational  evidence,  it  is 
only  necessary  to  keep  up  a  continual  series  of 
latitude  determinations  at  several  observatories. 
These  determinations  should  show  small  varia- 
tions similar  in  magnitude  to  the  wabblings  of  the 
pole. 

Let  us  now  consider  for  a  moment  what  is 
meant  by  the  axis  of  the  earth.  It  has  long  been 
known  that  the  planet  has  in  general  the  shape  of 
a  ball  or  sphere.  That  this  is  so  can  be  seen  at 

134 


MOTIONS   OF   THE   EARTH'S   POLE 

once  from  the  way  ships  at  sea  disappear  at  the 
horizon.  As  they  go  farther  and  farther  from  us, 
we  first  lose  sight  of  the  hull,  and  then  slowly 
and  gradually  the  spars  and  sails  seem  to  sink 
down  into  the  ocean.  This  proves  that  the 
earth's  surface  is  curved.  That  it  is  more  or  less 
like  a  sphere  is  evident  from  the  fact  that  it  always 
casts  a  round  shadow  in  eclipses.  Sometimes  the 
earth  passes  between  the  sun  and  eclipsed  moon. 
Then  we  see  the  earth's  black  shadow  projected 
on  the  moon,  which  would  otherwise  be  quite 
bright.  This  shadow  has  been  observed  in  a  very 
large  number  of  such  eclipses,  and  it  has  always 
been  found  to  have  a  circular  edge. 

While,  therefore,  the  earth  is  nearly  a  round 
ball,  it  must  not  be  supposed  that  it  is  exactly 
spherical  in  form.  We  may  disregard  the  small 
irregularities  of  its  surface,  for  even  the  greatest 
mountains  are  insignificant  in  height  when  com- 
pared with  the  entire  diameter  of  the  earth  itself. 
But  even  leaving  these  out  of  account,  the  earth  is 
not  perfectly  spherical.  We  can  describe  it  best 
as  a  flattened  sphere.  It  is  as  though  one  were 
to  press  a  round  rubber  ball  between  two  smooth 

135 


MOTIONS   OF   THE   EARTH'S   POLE 

boards.  It  would  be  flattened  at  the  top  and 
bottom  and  bulged  out  in  the  middle.  This  is 
the  shape  of  the  earth.  It  is  flattened  at  the 
poles  and  bulges  out  near  the  equator.  The 
shortest  straight  line  that  can  be  drawn  through 
the  earth's  centre  and  terminated  by  the  flattened 
parts  of  its  surface  may  be  called  the  earth's  axis 
of  figure ;  and  the  two  points  where  this  axis 
meets  the  surface  are  called  the  poles  of  figure. 

But  the  earth  has  another  axis,  called  the  axis 
of  rotation.  This  is  the  one  about  which  the 
planet  turns  once  in  a  day,  giving  rise  to  the  well- 
known  phenomena  called  the  rising  and  setting  of 
sun,  moon,  and  stars.  For  these  motions  of  the 
heavenly  bodies  are  really  only  apparent  ones, 
caused  by  an  actual  motion  of  the  observer  on 
the  earth.  The  observer  turns  with  the  earth  on 
its  axis,  and  is  thus  carried  past  the  sun  and  stars. 

This  daily  turning  of  the  earth,  then,  takes 
place  about  the  axis  of  rotation.  Now,  it  so  hap- 
pens that  all  kinds  of  astronomical  observations 
for  the  determination  of  latitude  lead  to  values 
based  on  the  rotation  axis  of  the  earth,  and  not 
on  its  axis  of  figure.  We  have  seen  how  the 

136 


MOTIONS   OF   THE   EARTH'S   POLE 

earth's  equator,  from  which  we  count  our  lati- 
tudes, is  everywhere  90  degrees  distant  from  the 
pole.  But  this  pole  is  the  pole  of  rotation,  or 
the  point  at  which  the  rotation  axis  pierces  the 
earth's  surface.  It  is  not  the  pole  of  figure. 

It  is  clear  that  the  latitude  of  any  observa- 
tory will  remain  constant  only  if  the  pole  of 
figure  and  the  rotation  pole  maintain  absolutely 
the  same  positions  relatively  one  to  the  other. 
These  two  poles  are  actually  very  near  together ; 
indeed,  it  was  supposed  for  a  very  long  time  that 
they  were  absolutely  coincident,  so  that  there  could 
not  be  any  variations  of  latitude.  But  it  now 
appears  that  they  are  separated  slightly. 

Strange  to  say,  one  of  them  is  revolving 
about  the  other  in  a  little  curve.  The  pole  of 
figure  is  travelling  around  the  pole  of  rotation. 
The  distance  between  them  varies  a  little,  never 
becoming  greater  than  about  fifty  feet,  and  it 
takes  about  fourteen  months  to  complete  a  revo- 
lution. There  are  some  slight  irregularities  in 
the  motion,  but,  in  the  main,  it  takes  place  in  the 
manner  here  stated.  In  consequence  of  this  ro- 
tation of  the  one  pole  about  the  other,  the  pole 

137 


MOTIONS   OF   THE   EARTH'S   POLE 

of  figure  is  now  on  one  side  of  the  rotation  pole 
and  now  on  the  opposite  side,  but  it  never  travels 
continuously  in  one  direction.  Thus,  as  we  have 
already  seen,  the  sort  of  continuous  motion  re- 
quired to  explain  the  observed  geological  phe- 
nomena has  not  yet  been  found  by  astronomers. 

Observations  for  the  study  of  latitude  varia- 
tions have  been  made  very  extensively  within 
recent  years  both  in  Europe  and  the  United 
States.  It  has  been  found  practically  most  ad- 
vantageous to  carry  out  simultaneous  series  of 
observations  at  two  observatories  situated  in 
widely  different  parts  of  the  earth,  but  having 
very  nearly  the  same  latitude.  It  is  then  pos- 
sible to  employ  the  same  stars  for  observation  in 
both  places,  whereas  it  would  be  necessary  to 
use  different  sets  of  stars  if  there  were  much 
difference  in  the  latitudes. 

There  is  a  special  advantage  in  using  the  same 
stars  in  both  places.  We  can  then  determine 
the  small  difference  in  latitude  between  the  two 
participating  observatories  in  a  manner  which  will 
make  it  quite  free  from  any  uncertainty  in  our 
knowledge  of  the  positions  on  the  sky  of  the 

138 


MOTIONS   OF   THE   EARTH'S   POLE 

stars  observed  ;  for,  strange  as  it  may  seem,  our 
star-catalogues  do  not  contain  absolutely  accurate 
numbers.  Like  all  other  data  depending  on 
fallible  human  observation,  they  are  affected  with 
small  errors.  But  if  we  can  determine  simply 
the  difference  in  latitude  of  the  two  observatories, 
we  can  discover  from  its  variation  the  path  in 
which  the  pole  is  moving.  If,  for  instance,  the 
observatories  are  separated  by  one-quarter  the 
circumference  of  the  globe,  the  pole  will  be  mov- 
ing directly  toward  one  of  them,  when  it  is  not 
changing  its  distance  from  the  other  one  at  all. 

This  method  was  usedNfor  seven  years  with  good 
effect  at  the  observatories  of  Columbia  Univer- 
sity in  New  York,  and  the  Royal  Observatory  at 
Naples,  Italy.  For  obtaining  its  most  complete 
advantages  it  is,  of  course,  better  to  establish  sev- 
eral observing  stations  on  about  the  same  parallel 
of  latitude.  This  was  done  in  1899  by  the  Inter- 
national Geodetic  Association.  Two  stations  are  in 
the  United  States,  one  in  Japan,  and  one  in  Sicily. 
We  can,  therefore,  hope  confidently  that  our  knowl- 
edge as  to  the  puzzling  problem  of  polar  motion 
will  soon  receive  very  material  advancement. 

139 


SATURN'S    RINGS 

THE  death  of  James  E.  Keeler,  Director  of  the 
Lick  Observatory,  in  California  (p.  32),  recalls 
to  mind  one  of  the  most  interesting  and  signifi- 
cant of  later  advances  in  astronomical  science. 
Only  seven  years  have  elapsed  since  Keeler  made 
the  remarkable  spectroscopic  observations  which 
gave  for  the  first  time  an  ocular  demonstration  of 
the  true  character  of  those  mysterious  luminous 
rings  surrounding  the  brilliant  planet  Saturn. 
His  results  have  not  yet  been  made  sufficiently 
accessible  to  the  public  at  large,  nor  have  they 
been  generally  valued  at  their  true  worth.  We 
consider  this  work  of  Keeler's  interesting,  be- 
cause the  problem  of  the  rings  has  been  a  classic 
one  for  many  generations  ;  and  we  have  been 
particular,  also,  to  call  it  significant,  because  it  is 
pregnant  with  the  possibilities  of  newer  methods 
of  spectroscopic  research,  applied  in  the  older 

departments  of  observational  astronomy. 

140 


SATURN'S   RINGS 

The  troubles  of  astronomers  with  the  rings 
began  with  the  invention  of  the  telescope  itself. 
They  date  back  to  1610,  when  Galileo  first 
turned  his  new  instrument  to  the  heavens  (p.  49). 
It  may  be  imagined  easily  that  the  bright  planet 
Saturn  was  among  the  very  first  objects  scruti- 
nized by  him.  His  "  powerful "  instrument 
magnified  only  about  thirty  times,  and  was, 
doubtless,  much  inferior  to  our  pocket  telescopes 
of  to-day.  But  it  showed,  at  all  events,  that 
something  was  wrong  with  Saturn.  Galileo  put 
it,  "Uliimam  planetam  tergeminam  observavi" 
("  I  have  observed  the  furthest  planet  to  be 
triple  " ). 

It  is  easy  to  understand  now  how  Galileo's 
eyes  deceived  him.  For  a  round  luminous  ball 
like  Saturn,  surrounded  by  a  thin  flat  ring  seen 
nearly  edgewise,  really  looks  as  if  it  had  two  lit- 
tle attached  appendages.  Strange,  indeed,  it  is 
to-day  to  read  a  scientific  book  so  old  that  the 
planet  Saturn  could  be  called  the  "  furthest " 
planet.  But  it  was  the  outermost  known  in 
Galileo's  day,  and  for  nearly  two  centuries  after- 
ward. Not  until  1781  did  William  Herschel 

141 


SATURN'S   RINGS 

discover  Uranus  (p.  59)  ;  and  Neptune  was  not 
disclosed  by  the  marvellous  mathematical  percep- 
tion of  Le  Verrier  until  1846  (p.  61). 

Galileo's  further  observations  of  Saturn  both- 
ered him  more  and  more.  The  planet's  behav- 
ior became  much  worse  as  time  went  on.  "  Has 
Saturn  devoured  his  children,  according  to  the 
old  legend  ? "  he  inquired  soon  afterward ;  for 
the  changed  positions  of  earth  and  planet  in  the 
course  of  their  motions  around  the  sun  in  their 
respective  orbits  had  become  such  that  the  ring 
was  seen  quite  edgewise,  and  was,  therefore,  per- 
fectly invisible  to  Galileo's  "  optic  tube."  The 
puzzle  remained  unsolved  by  Galileo ;  it  was  left 
for  another  great  man  to  find  the  true  answer. 
Huygens,  in  1656,  first  announced  that  the  ring 
is  a  ring. 

The  manner  in  which  this  announcement  was 
made  is  characteristic  of  the  time ;  to-day  it 
seems  almost  ludicrous.  Huygens  published  a 
little  pamphlet  in  1656  called  "  De  Saturni  Luna 
Observatio  Nova"  or,  "  A  New  Observation  of 
Saturn's  Moon."  He  gave  the  explanation  of 

what  had  been  observed  by  himself  and  preced- 

142 


Or 

UNi\ 


SATURN'S   RINGS 

ing  astronomers  in  the  form  of  a  puzzle,  or 
"  logogriph."  Here  is  what  he  had  to  say  of 
the  phenomenon  in  question : 

"aaaaaaa  ccccc  d  eeeee  g  h  iiiiiii  1111  mm 
nnnnnnnnn  oooo  pp  q  rr  s  ttttt  uuuuu." 

It  was  not  until  1659,  three  years  later,  in  a 
book  entitled  " Systema  Saturnium"  that  Huy- 
gens  rearranged  the  above  letters  in  their  proper 
order,  giving  the  Latin  sentence  : 

"  Annulo  cingitur,  tenui  piano,  nusquam  co- 
haerente,  ad  eclipticam  inclinato"  Translated 
into  English,  this  sentence  informs  us  that  the 
planet  "  is  girdled  with  a  thin,  flat  ring,  nowhere 
touching  Saturn,  and  inclined  to  the  ecliptic  "  ! 

This  was  a  perfectly  correct  and  wonderfully 
sagacious  explanation  of  those  complex  and  exas- 
peratingly  puzzling  phenomena  that  had  been 
too  difficult  for  no  less  a  person  than  Galileo 
himself.  It  was  an  explanation  that  explained. 
The  reason  for  its  preliminary  announcement  in 
the  above  manner  must  have  been  the  following : 
Huygens  was  probably  not  quite  sure  of  his 
ground  in  1656,  while  three  years  afterward  he 
had  become  quite  certain.  By  the  publication  of 

143 


SATURN'S  RINGS 

the  logogriph  of  1656  he  secured  for  himself  the 
credit  of  what  he  had  done.  If  any  other  as- 
tronomer had  published  the  true  explanation 
after  1656,  Huygens  could  have  proved  his 
claim  to  priority  by  rearranging  the  letters  of  his 
puzzle.  On  the  other  hand,  if  further  researches 
showed  him  that  he  was  wrong,  he  would  never 
have  made  known  the  true  meaning  of  his  logo- 
griph, and  would  thus  have  escaped  the  igno- 
miny of  making  an  erroneous  explanation.  Thus, 
the  method  of  announcement  was  comparable 
in  ingenuity  with  the  Huygenian  explanation 
itself. 

We  are  compelled  to  pass  over  briefly  the  en- 
tertaining history  of  subsequent  observations  of 
the  ring,  in  order  to  explain  the  new  work  of 
Keeler  and  others.  Cassini,  about  1675,  nad 
been  able  to  show  that  the  ring  was  double ;  that 
there  are  really  two  independent  rings,  with  a 
distinct  dark  space  between  them.  It  was  a  case 
of  wheels  within  wheels.  To  our  own  eminent 
countryman,  W.  C.  Bond,  of  Cambridge,  Mass., 
we  owe  the  further  discovery  (Harvard  College 
Observatory,  November,  1850)  of  the  third 

144 


SATURN'S   RINGS 

ring.  This  is  also  concentric  with  the  other  two, 
and  interior  to  them,  but  difficult  to  observe, 
because  of  its  much  smaller  luminosity. 

It  is  almost  transparent,  and  the  brilliant  light 
of  the  planet's  central  ball  is  capable  of  shining 
directly  through  it.  For  this  reason  the  inner 
ring  is  called  the  "  gauze  "  or  "  crape  "  ring.  If 
we  add  to  the  above  details  the  fact  that  our 
modern  large  telescopes  show  slight  irregularities 
in  the  surface  of  the  rings,  especially  when  seen 
edgewise,  we  have  a  brief  statement  of  all  that 
the  telescope  has  been  able  to  reveal  to  us  since 
Galileo's  time. 

But  of  far  greater  interest  than  the  mere  fact 
of  their  existence  is  the  important  cosmic  ques- 
tion as  to  the  constitution,  structure,  and,  above 
all,  durability  of  the  ring  system.  Astronomers 
often  use  the  term  "  stability "  with  regard  to 
celestial  systems  like  the  ring  system  of  Saturn. 
By  this  they  mean  permanent  durability.  A 
system  is  stable  if  its  various  parts  can  continue 
in  their  present  relationship  to  one  another,  with- 
out violating  any  of  the  known  laws  of  astron- 
omy. Whenever  we  study  any  collection  of 


SATURN'S   RINGS 

celestial  objects,  and  endeavor  to  explain  their 
motions  and  peculiarities,  we  always  seek  some 
explanation  not  inconsistent  with  the  continued 
existence  of  the  phenomena  in  question.  For 
this  there  is,  perhaps,  no  sufficient  philosophical 
basis.  Probably  much  of  the  great  celestial  pro- 
cession is  but  a  passing  show,  to  be  but  for  a 
moment  in  the  endless  vista  of  cosmic  time. 

However  this  may  be,  we  are  bound  to  as- 
sume as  a  working  theory  that  Saturn  has  always 
had  these  rings,  and  will  always  have  them ;  and 
it  is  for  us  to  find  out  how  this  is  possible.  The 
problem  has  been  attacked  mathematically  by 
various  astronomers,  including  Laplace  ;  but  no 
conclusive  mathematical  treatment  was  obtained 
until  1857,  when  James  Clark-Maxwell  proved 
in  a  masterly  manner  that  the  rings  could  be 
neither  solid  nor  liquid.  He  showed,  indeed, 
that  they  would  not  last  if  they  were  continuous 
bodies  like  the  planets.  A  big  solid  wheel 
would  inevitably  be  torn  asunder  by  any  slight 
disturbance,  and  then  precipitated  upon  the  plan- 
et's surface.  Therefore,  the  rings  must  be  com- 
posed of  an  immense  number  of  small  detached 

146 


SATURN'S   RINGS  • 

particles,  revolving  around  Saturn  in  separate 
orbits,  like  so  many  tiny  satellites. 

This  mathematical  theory  of  the  ring  system 
being  once  established,  astronomers  were  more 
eager  than  ever  to  obtain  a  visual  confirmation 
of  it.  We  had,  indeed,  a  sort  of  analogy  in 
the  assemblage  of  so-called  "  minor  planets " 
(p.  64),  which  are  known  to  be  revolving  around 
our  sun  in  orbits  situated  between  Mars  and 
Jupiter.  Some  hundreds  of  these  are  known 
to  exist,  and  probably  there  are  countless  others 
too  small  for  us  to  see.  Such  a  swarm  of  tiny 
particles  of  luminous  matter  would  certainly 
give  the  impression  of  a  continuous  solid  body, 
if  seen  from  a  distance  comparable  to  that  sepa- 
rating us  from  Saturn.  But  arguments  founded 
on  analogy  are  of  comparatively  little  value. 

Astronomers  need  direct  and  conclusive  tele- 
scopic evidence,  and  this  was  lacking  until  Keeler 
made  his  remarkable  spectroscopic  observation  in 
1895.  The  spectroscope  is  a  peculiar  instru- 
ment, different  in  principle  from  any  other  used 
in  astronomy  ;  we  study  distant  objects  with  it 
by  analyzing  the  light  they  send  us,  rather  than 

147 


SATURN'S  RINGS 

by  examining  and  measuring  the  details  of  their 
visible  surfaces.  The  reader  will  recall  that  ac- 
cording to  the  modern  undulatory  theory,  light 
consists  simply  of  a  series  of  waves.  Now,  the 
nature  of  waves  is  very  far  from  being  under- 
stood in  the  popular  mind.  Most  people,  for 
instance,  think  that  the  waves  of  ocean  consist  of 
great  masses  of  water  rolling  along  the  surface. 

This  notion  doubtless  arises  from  the  behavior 
of  waves  when  they  break  upon  the  shore,  form- 
ing what  we  call  surf.  When  a  wave  meets 
with  an  immovable  body  like  a  sand  beach,  the 
wave  is  broken,  and  the  water  really  does  roll 
upon  the  beach.  But  this  is  an  exceptional  case. 
Farther  away  from  the  shore,  where  the  waves 
are  unimpeded,  they  consist  simply  of  particles 
of  water  moving  straight  up  and  down.  None 
of  the  water  is  carried  by  mere  wave-action  away 
from  the  point  over  which  it  was  situated  at  first. 

Tides  or  other  causes  may  move  the  water,  but 
not  simple  wave-motion  alone.  That  this  is  so 
can  be  proved  easily.  If  a  chip  of  wood  be 
thrown  overboard  from  a  ship  at  sea  it  will  be 

seen  to  rise  and  fall  a  long  time  on  the  waves, 

148 


SATURN'S   RINGS 

but  it  will  not  move.  Similarly,  wind-waves  are 
often  quite  conspicuous  on  a  field  of  grain ;  but 
they  are  caused  by  the  individual  grain  particles 
moving  up  and  down.  The  grain  certainly  can- 
not travel  over  the  ground,  since  each  particle  is 
fast  to  its  own  stalk. 

But  while  the  particles  do  not  travel,  the  wave- 
disturbance  does.  At  times  it  is  transmitted  to 
a  considerable  distance  from  the  point  where  it 
was  first  set  in  motion.  Thus,  when  a  stone  is 
dropped  into  still  water,  the  disturbance  (though 
not  the  water)  travels  in  ever-widening  circles, 
until  at  last  it  becomes  too  feeble  for  us  to  per- 
ceive. Light  is  just  such  a  travelling  wave-dis- 
turbance. Beginning,  perhaps,  in  some  distant 
star,  it  travels  through  space,  and  finally  the 
wave  impinges  on  our  eyes  like  the  ocean-wave 
breaking  on  a  sand  beach.  Such  a  light-wave 
affects  the  eye  in  some  mysterious  way.  We 
call  it  cc  seeing." 

The  spectroscope  (p.  21)  enables  us  to  meas- 
ure and  count  the  waves  reaching  us  each  second 
from  any  source  of  light.  No  matter  how  far 

away  the  origin  of  stellar  light  may  be,  the  spec- 

149 


SATURN'S   RINGS 

troscope  examines  the  character  of  that  light,  and 
tells  us  the  number  of  waves  set  up  every  sec- 
ond. It  is  this  characteristic  of  the  instrument 
that  has  enabled  us  to  make  some  of  the  most 
remarkable  observations  of  modern  times.  If 
the  distant  star  is  approaching  us  in  space,  more 
light -waves  per  second  will  reach  us  than  we 
should  receive  from  the  same  star  at  rest.  Thus 
if  we  find  from  the  spectroscope  that  there  are 
too  many  waves,  we  know  that  the  star  is  com- 
ing nearer ;  and  if  there  are  too  few,  we  can 
conclude  with  equal  certainty  that  the  star  is 
receding. 

Keeler  was  able  to  apply  the  spectroscope  in 
this  way  to  the  planet  Saturn  and  to  the  ring 
system.  The  observations  required  dexterity 
and  observational  manipulative  skill  in  a  superla- 
tive degree.  These  Keeler  had  ;  and  this  work 
of  his  will  always  rank  as  a  classic  observation. 
He  found  by  examining  the  light-waves  from 
opposite  sides  of  the  planet  that  the  luminous 
ball  rotated ;  for  one  side  was  approaching  us 
and  the  other  receding.  This  observation  was, 
of  course,  in  accord  with  the  known  fact  of  Sat- 

150 


SATURN'S   RINGS 

urn's  rotation  on  his  axis.  With  regard  to  the 
rings,  Keeler  showed  in  the  same  way  the  ex- 
istence of  an  axial  rotation,  which  appears  not 
to  have  been  satisfactorily  proved  before,  strange 
as  it  may  seem.  But  the  crucial  point  estab- 
lished by  his  spectroscope  was  that  the  interior 
part  of  the  rings  rotates  faster  than  the  exterior. 

The  velocity  of  rotation  diminishes  gradually 
from  the  inside  to  the  outside.  This  fact  is  ab- 
solutely inconsistent  with  the  motion  of  a  solid 
ring ;  but  it  fits  in  admirably  with  the  theory  of 
a  ring  comprised  of  a  vast  assemblage  of  small 
separate  particles.  Thus,  for  the  first  time,  as- 
tronomy comes  into  possession  of  an  observa- 
tional determination  of  the  nature  of  Saturn's 
rings,  and  Galileo's  puzzle  is  forever  solved. 


151 


THE    HBLIOMETER 


ASTRONOMICAL  discoveries  are  always  received 
by  the  public  with  keen  interest.  Every  new 
fact  read  in  the  great  open  book  of  nature  is  writ- 
ten eagerly  into  the  books  of  men.  For  there 
exists  a  strong  curiosity  to  ascertain  just  how  the 
greater  world  is  built  and  governed ;  and  it  must 
be  admitted  that  astronomers  have  been  able  to 
satisfy  that  curiosity  with  no  small  measure  of 
success.  But  it  is  seldom  that  we  hear  of  the 
means  by  which  the  latest  and  most  refined 
astronomical  observations  are  effected.  Popular 
imagination  pictures  the  astronomer,  as  he  doubt- 
less once  was,  an  aged  gentleman,  usually  having 
a  long  white  beard,  and  spending  entire  nights 
staring  at  the  sky  through  a  telescope. 

But  the  facts  to-day  are  very  different.  The 
working  astronomer  is  an  active  man  in  the 
prime  of  life,  often  a  young  man.  He  wastes  no 
time  in  star-gazing.  His  observations  consist  of 

152 


THE   HELIOMETER 

exact  measurements  made  in  a  precise,  systemat- 
ic, and  almost  business-like  manner.  A  night's 
"  watch "  at  the  telescope  is  seldom  allowed  to 
exceed  about  three  hours,  since  it  is  found  that 
more  continued  exertions  fatigue  the  eye  and  lead 
to  less  accurate  results.  To  this,  of  course,  there 
have  been  many  notable  exceptions,  for  endurance 
of  sight,  like  any  form  of  physical  strength,  differs 
greatly  in  different  individuals.  Astronomical  re- 
search does  not  include  "  picking  out "  the  con- 
stellations, and  learning  the  Arabic  names  of  in- 
dividual stars.  These  things  are  not  without 
interest ;  but  they  belong  to  astronomy's  ancient 
history,  and  are  of  little  value  except  to  afford 
amusement  and  instruction  to  successive  genera- 
tions of  amateurs. 

Among  the  instruments  for  carefully  planned 
measurements  of  precision  the  heliometer  prob- 
ably takes  first  rank.  It  is  at  once  the  most 
exquisitely  accurate  in  its  results,  and  the  most 
fatiguing  to  the  observer,  of  all  the  varied  ap- 
paratus employed  by  the  astronomer.  The  prin- 
ciple upon  which  its  construction  depends  is  very 
peculiar,  and  applies  to  all  telescopes,  even  or- 

153 


THE   HELIOMETER 

dinary  ones  for  terrestrial  purposes.  If  part  of  a 
telescope  lens  be  covered  up  with  the  hand,  it 
will  still  be  possible  to  see  through  the  instru- 
ment. The  glass  lens  at  the  end  of  the  tube 
farthest  from  the  observer's  eye  helps  to  magnify 
distant  objects  and  make  them  seem  nearer  by 
gathering  to  a  single  point,  or  focus,  a  greater 
amount  of  their  light  than  could  be  brought 
together  by  the  far  smaller  lens  in  the  unaided 
eye. 

The  telescope  might  very  properly  be  likened 
to  an  enlarged  eye,  which  can  see  more  than  we 
can,  simply  because  it  is  bigger.  If  a  telescope 
lens  has  a  surface  one  hundred  times  as  large  as 
that  of  the  lens  in  our  eye,  it  will  gather  and  bring 
to  a  focus  one  hundred  times  as  much  light  from  a 
distant  object.  Now,  if  any  part  of  this  telescope 
be  covered,  the  remaining  part  will,  nevertheless, 
gather  and  focus  light  just  as  though  the  whole 
lens  were  in  action  ;  only,  there  will  be  less  light 
collected  at  the  focus  within  the  tube.  The 
small  lens  at  the  telescope's  eye-end  is  simply  a 
magnifier  to  help  our  eye  examine  the  image  of 
any  distant  object  formed  at  the  focus  by  the 

154 


THE   HELIOMETER 

large  lens  at  the  farther  end  of  the  instrument. 
For  of  this  simple  character  is  the  operation  of 
any  telescope :  the  large  glass  lens  at  one  end 
collects  a  distant  planet's  light,  and  brings  it  to  a 
focus  near  the  other  end  of  the  tube,  where  it 
forms  a  tiny  picture  of  the  planet,  which,  in 
turn,  is  examined  with  the  little  magnifier  at 
the  eye-end. 

Having  arrived  at  the  fundamental  principle 
that  part  of  a  lens  will  act  in  a  manner  similar  to 
a  whole  one,  it  is  easy  to  explain  the  construction 
of  a  heliometer.  An  ordinary  telescope  lens  is 
sawed  in  half  by  means  of  a  thin  round  metal 
disk  revolved  rapidly  by  machinery,  and  fed  con- 
tinually with  emery  and  water  at  its  edge.  The 
cutting  effect  of  emery  is  sufficient  to  make  such 
a  disk  enter  glass  much  as  an  ordinary  saw  pene- 
trates wood.  The  two  "  semi-lenses,"  as  they 
are  called,  are  then  mounted  separately  in  metal 
holders.  These  are  attached  to  one  end  of  the 
heliometer,  called  the  "  head,"  in  such  a  way  that 
the  two  semi-lenses  can  slide  side  by  side  upon 
metal  guides.  This  head  is  then  fastened  to  one 
end  of  a  telescope  tube  mounted  in  the  usual 

155 


THE   HELIOMETER 

way.  The  "  head "  end  of  the  instrument  is 
turned  toward  the  sky  in  observing,  and  at  the 
eye-end  is  placed  the  usual  little  magnifier  we 
have  already  described. 

The  observer  at  the  eye-end  has  control  of 
certain  rods  by  means  of  which  he  can  push  the 
semi-lenses  on  their  slides  in  the  head  at  the 
other  end  of  the  tube.  Now,  if  he  moves  the 
semi-lenses  so  as  to  bring  them  side  by  side  ex- 
actly, the  whole  arrangement  will  act  like  an  or- 
dinary telescope.  For  the  semi-lenses  will  then 
fit  together  just  as  if  the  original  glass  had  never 
been  cut.  But  if  the  half-lenses  are  separated  a 
little  on  their  slides,  each  will  act  by  itself.  Be- 
ing slightly  separated,  each  will  cover  a  different 
part  of  the  sky.  The  whole  affair  acts  as  if  the 
observer  at  the  eye-end  were  looking  through 
two  telescopes  at  once.  For  each  semi-lens  acts 
independently,  just  as  if  it  were  a  complete  glass 
of  only  half  the  size. 

Now,  suppose  there  were  a  couple  of  stars  in 
the  sky,  one  in  the  part  covered  by  the  first 
semi-lens,  and  one  in  the  part  covered  by  the 
second.  The  observer  would,  of  course,  see 

156 


THE   HELIOMETER 

both  stars  at  once  upon   looking  into   the  little 
magnifier  at  the  eye-end  of  the  heliometer. 

We  must  remember  that  these  stars  will 
appear  in  the  field  of  view  simply  as  two  tiny 
points  of  light.  The  astronomer,  as  we  have 
said,  is  provided  with  a  simple  system  of  long 
rods,  by  means  of  which  he  can  manipulate  the 
semi-lenses  while  the  observation  is  being  made. 
If  he  slides  them  very  slowly  one  way  or  the 
other,  the  two  star-points  in  the  field  of  view  will 
be  seen  to  approach  each  other.  In  this  way 
they  can  at  last  be  brought  so  near  together  that 
they  will  form  but  a  single  dot  of  light.  Obser- 
vation with  the  heliometer  consists  in  thus  bring- 
ing two  star-images  together,  until  at  last  they 
are  superimposed  one  upon  the  other,  and  we  see 
one  image  only.  Means  are  provided  by  which 
it  is  then  possible  to  measure  the  amount  of 
separation  of  the  two  half-lenses.  Evidently  the 
farther  asunder  on  the  sky  are  the  two  stars 
under  observation,  the  greater  will  be  the  separa- 
tion of  the  semi-lenses  necessary  to  make  a  single 
image  of  their  light.  Thus,  measurement  of  the 
lenses'  separation  becomes  a  means  of  determin- 

157 


THE   HELIOMETER 

ing  the  separation  of  the  stars  themselves  upon 
the  sky. 

The  two  slides  in  the  heliometer  head  are  sup- 
plied with  a  pair  of  very  delicate  measures  or 
"  scales  "  usually  made  of  silver.  These  can  be 
examined  from  the  eye-end  of  the  instrument  by 
looking  through  a  long  microscope  provided  for 
this  special  purpose.  Thus  an  extremely  precise 
value  is  obtained  both  of  the  separation  of  the 
sliders  and  of  the  distance  on  the  sky  between 
the  stars  under  examination.  Measures  made  in 
this  way  with  the  heliometer  are  counted  the 
most  precise  of  astronomical  observations. 

Having  thus  described  briefly  the  kind  of  ob- 
servations obtained  with  the  heliometer,  we  shall 
now  touch  upon  their  further  utilization.  We 
shall  take  as  an  example  but  one  of  their  many 
uses — that  one  which  astronomers  consider  the 
most  important — the  measurement  of  stellar  dis- 
tances. (See  also  p.  94.) 

Even  the  nearest  fixed  star  is  almost  incon- 
ceivably remote  from  us.  And  astronomers 
are  imprisoned  on  this  little  earth ;  we  cannot 
bridge  the  profound  distance  separating  us  from 

158 


THE   HELIOMETER 

the  stars,  so  as  to  use  direct  measurement  with 
tape-line  or  surveyor's  chain.  We  are  forced  to 
have  recourse  to  some  indirect  method.  Suppose 
a  certain  star  is  suspected,  on  account  of  its  bright- 
ness, or  for  some  other  reason,  of  being  near  us 
in  space,  and  so  giving  a  favorable  opportunity 
for  a  determination  of  distance.  A  couple  of 
very  faint  stars  are  selected  close  by.  These,  on 
account  of  their  faintness,  the  astronomer  may 
regard  as  quite  immeasurably  far  away.  He  then 
determines  with  his  heliometer  the  exact  position 
on  the  sky  of  the  bright  star  with  respect  to  the 
pair  of  faint  ones.  Half  a  year  is  then  allowed 
to  pass.  During  that  time  the  earth  has  been 
swinging  along  in  its  annual  path  or  orbit  around 
the  sun.  Half  a  year  will  have  sufficed  to  carry 
the  observer  on  the  earth  to  the  other  side  of 
that  path,  and  he  is  then  185,000,000  miles  away 
from  his  position  at  the  first  observation. 

Another  determination  is  made  of  the  bright 
star's  position  as  referred  to  the  two  faint  ones. 
Now,  if  all  these  stars  were  equally  distant,  their 
relative  positions  at  the  second  observation  would 
be  just  the  same  as  at  the  former  one.  But  if,  as 

159 


THE   HELIOMETER 

is  very  probable,  the  bright  star  is  very  much 
nearer  us  than  are  the  two  faint  ones,  we  shall 
obtain  a  different  position  from  our  second  ob- 
servation. For  the  change  of  185,000,000  miles 
in  the  observer's  location  will,  of  course,  affect 
the  direction  in  which  we  see  the  near  star,  while 
it  will  leave  the  distant  ones  practically  unchanged. 
Without  entering  into  technical  details,  we  may 
say  that  from  a  large  number  of  observations  of 
this  kind,  we  can  obtain  the  distance  of  the  bright 
star  by  a  process  of  calculation.  The  only  es- 
sential is  to  have  an  instrument  that  can  make 
the  actual  observations  of  position  accurately 
enough ;  and  in  this  respect  the  heliometer  is  still 
unexcelled. 


T  60 


OCCULTATIONS 

SCARCELY  anyone  can  have  watched  the  sky 
without  noticing  how  different  is  the  behavior  of 
our  moon  from  that  of  any  other  object  we  can 
see.  Of  course,  it  has  this  in  common  with  the 
sun  and  stars  and  planets,  that  it  rises  in  the 
eastern  horizon,  slowly  climbs  the  dome  of  the 
sky,  and  again  goes  down  and  sets  in  the  west. 
This  motion  of  the  heavenly  bodies  is  known  to 
be  an  apparent  one  merely,  and  caused  by  the 
turning  of  our  own  earth  upon  its  axis.  A  man 
standing  upon  the  earth's  surface  can  look  up 
and  see  above  him  one-half  the  great  celestial 
vault,  gemmed  with  twinkling  stars,  and  bearing, 
perhaps,  within  its  ample  curve  one  or  two  se- 
renely shining  planets  and  the  lustrous  moon. 
But  at  any  given  moment  the  observer  can  see 
nothing  of  the  other  half  of  the  heavenly  sphere. 
It  is  beneath  his  feet,  and  concealed  by  the  solid 
bulk  of  the  earth. 

161 


OCCULTATIONS 

The  earth,  however,  is  turning  on  an  axis, 
carrying  the  observer  with  it.  And  so  it  is  con- 
tinually presenting  him  to  a  new  part  of  the  sky. 
At  any  moment  he  sees  but  a  single  half-sphere ; 
yet  the  very  next  instant  it  is  no  longer  the  same ; 
a  small  portion  has  passed  out  of  sight  on  one 
side  by  going  down  behind  the  turning  earth, 
while  a  corresponding  new  section  has  come  into 
view  on  the  opposite  side.  It  is  this  coming  into 
view  that  we  call  the  rising  of  a  star ;  and  the 
corresponding  disappearance  on  the  other  side  is 
called  setting.  Thus  rising  and  setting  are,  of 
course,  due  entirely  to  a  turning  of  the  earth,  and 
not  at  all  to  actual  motions  of  the  stars ;  and  for 
this  reason,  all  objects  in  the  sky,  without  ex- 
ception, must  rise  and  set  again.  But  the  moon 
really  has  a  motion  of  its  own  in  addition  to  this 
apparent  one  caused  by  the  earth's  rotation. 

Somewhere  in  the  dawn  of  time  early  watchers 
of  the  stars  thought  out  those  fancied  constella- 
tions that  survive  even  down  to  our  own  day. 
They  placed  the  mighty  lion,  king  of  beasts, 
upon  the  face  of  night,  and  the  great  hunter,  too, 
armed  with  club  and  dagger,  to  pursue  him. 

162 


OCCULTATIONS 

Among  these  constellations  the  moon  threads  her 
destined  way,  night  after  night,  so  rapidly  that 
the  unaided  eye  can  see  that  she  is  moving.  It 
needs  but  little  power  of  fancy's  magic  to  recall 
from  the  dim  past  a  picture  of  some  aged  astron- 
omer graving  upon  his  tablets  the  Records  of 
the  Moon.  "  To-night  she  is  near  the  bright 
star  in  the  eye  of  the  Bull."  And  again  :  "  To- 
night she  rides  full,  and  near  to  the  heart  of  the 
Virgin." 

And  why  does  the  moon  ride  thus  through  the 
stars  of  night  ?  Modern  science  has  succeeded 
in  disentangling  the  intricacies  of  her  motion, 
until  to-day  only  one  or  two  of  the  very  minutest 
details  of  that  motion  remain  unexplained.  But 
it  has  been  a  hard  problem.  Someone  has  well 
said  that  lunar  theory  should  be  likened  to  a 
lofty  cliff  upon  whose  side  the  intellectual  giants 
among  men  can  mark  off  their  mental  stature, 
but  whose  height  no  one  of  them  may  ever  hope 
to  scale. 

But  for  our  present  purpose  it  is  unnecessary 
to  pursue  the  subject  of  lunar  motion  into  its 
abstruser  details.  To  understand  why  the  moon 

163 


OCCULTATIONS 

moves  rapidly  among  the  stars,  it  is  sufficient  to 
remember  that  she  is  whirling  quickly  round  the 
earth,  so  as  to  complete  her  circuit  in  a  little  less 
than  a  month.  We  see  her  at  all  times  projected 
upon  the  distant  background  of  the  sky  on  which 
are  set  the  stellar  points  of  light,  as  though  in- 
tended for  beacons  to  mark  the  course  pursued 
by  moon  and  planets.  The  stars  themselves  have 
no  such  motions  as  the  moon ;  situated  at  a  dis- 
tance almost  inconceivably  great,  they  may,  in- 
deed, be  travellers  through  empty  space,  yet  their 
journeys  shrink  into  insignificance  as  seen  from 
distant  earth.  It  requires  the  most  delicate  in- 
struments of  the  astronomer  to  so  magnify  the 
tiny  displacements  of  the  stars  on  the  celestial 
vault  that  they  may  be  measured  by  human  eyes. 
Let  us  again  recur  to  our  supposed  observer 
watching  the  moon  night  after  night,  so  as  to 
record  the  stars  closely  approached  by  her.  Why 
should  he  not  some  time  or  other  be  surprised 
by  an  approach  so  close  as  to  amount  apparent- 
ly to  actual  contact  ?  The  moon  covers  quite 
a  large  surface  on  the  sky,  and  when  we  remem- 
ber the  nearly  countless  numbers  of  the  stars,  it 

164 


OCCULTATIONS 

would,  indeed,  be  strange  if  there  were  not  some 
behind  the  moon  as  well  as  all  around  her. 

A  moment's  consideration  shows  that  this  must 
often  be  the  case ;  and  in  fact,  if  the  moon's  ad- 
vancing edge  be  scrutinized  carefully  through  a 
telescope,  small  stars  can  be  seen  frequently  to 
disappear  behind  it.  This  is  a  most  interesting 
observation.  At  first  we  see  the  moon  and  star 
near  each  other  in  the  telescope's  field  of  view. 
But  the  distance  between  them  lessens  percepti- 
bly, even  quickly,  until  at  last,  with  a  startling 
suddenness,  the  star  goes  out  of  sight  behind  the 
moon.  After  a  time,  ranging  from  a  few  mo- 
ments to,  perhaps,  more  than  an  hour,  the  moon 
will  pass,  and  we  can  see  the  star  suddenly  reap- 
pear from  behind  the  other  edge. 

These  interesting  observations,  while  not  at  all 
uncommon,  can  be  made  only  very  rarely  by 
naked-eye  astronomers.  The  reason  is  simple. 
The  moon's  light  is  so  brilliant  that  it  fairly  over- 
comes the  stars  whenever  they  are  at  all  near,  ex- 
cept in  the  case  of  very  bright  ones.  Small 
stars  that  can  be  followed  quite  easily  up  to  the 
moon's  edge  in  a  good  telescope,  disappear  from 

165 


OCCULTATIONS 

a  naked-eye  view  while  the  moon  is  still  a  long 
distance  away.  But  the  number  of  very  bright 
stars  is  comparatively  small,  so  that  it  is  quite  un- 
usual to  find  anyone  not  a  professional  astrono- 
mer who  has  actually  seen  one  of  these  so-called 
"  occultations."  Moreover,  most  people  are  not 
informed  in  advance  of  the  occurrence  of  an  op- 
portunity to  make  such  observations,  although 
they  can  be  predicted  quite  easily  by  the  aid  of 
astronomical  calculations.  Sometimes  we  have 
occultations  of  planets,  and  these  are  the  most  in- 
teresting of  all.  When  the  moon  passes  between 
us  and  one  of  the  larger  planets,  it  is  worth 
while  to  observe  the  phenomenon  even  without  a 
telescope. 

Up  to  this  point  we  have  considered  occulta- 
tions chiefly  as  being  of  interest  to  the  naked-eye 
astronomer.  Yet  occultations  have  a  real  scien- 
tific value.  It  is  by  their  means  that  we  can, 
perhaps,  best  measure  the  moon's  size.  By  not- 
ing with  a  telescope  the  time  of  disappearance 
and  reappearance  of  known  stars,  astronomers 
can  bring  the  direct  measurement  of  the  moon's 
diameter  within  the  range  of  their  numerical  cal- 

166 


OCCULTATIONS 

dilations.  Sometimes  the  moon  passes  over  a 
condensed  cluster  of  stars  like  the  Pleiades. 
The  results  obtainable  on  these  occasions  are 
valuable  in  a  very  high  degree,  and  contribute 
largely  to  making  precise  our  knowledge  of  the 
lunar  diameter. 

There  is  another  thing  of  scientific  interest 
about  occultations,  though  it  has  lost  some  of 
its  importance  in  recent  years.  When  such  an 
event  has  been  observed,  the  agreement  of  the 
predicted  time  with  that  actually  recorded  by  the 
astronomer  offers  a  most  searching  test  of  the 
correctness  of  our  theory  of  lunar  motion.  We 
have  already  called  attention  to  the  great  inher- 
ent difficulty  of  this  theory.  It  is  easy  to  see 
that  by  noting  the  exact  instant  of  disappearance 
of  a  star  at  a  place  on  the  earth  the  latitude  and 
longitude  of  which  are  known,  we  can  both 
check  our  calculations  and  gather  material  for 
improving  our  theory.  The  same  principle  can 
be  used  also  in  the  converse  direction.  Within 
the  limits  of  precision  imposed  by  the  state  of 
our  knowledge  of  lunar  theory,  we  can  utilize 
occultations  to  help  determine  the  position  on  the 

167 


OCCULTATIONS 

earth  of  places  whose  longitude  is  unknown.  It 
is  a  very  interesting  bit  of  history  that  the  first 
determination  of  the  longitude  of  Washington 
was  made  by  means  of  occultations,  and  that  this 
early  determination  led  to  the  founding  of  the 
United  States  Naval  Observatory. 

On  March  28,  1810,  Mr.  Pitkin,  of  Connec- 
ticut, reported  to  the  House  of  Representatives 
on  "  laying  a  foundation  for  the  establishment  of 
a  first  meridian  for  the  United  States,  by  which 
a  further  dependence  on  Great  Britain  or  any 
other  foreign  nation  for  such  meridian  may  be 
entirely  removed."  This  report  was  the  result 
of  a  memorial  presented  by  one  William  Lam- 
bert, who  had  deduced  the  longitude  of  the  Cap- 
itol from  an  occultation  observed  October  20, 
1804.  Various  proceedings  were  had  in  Con- 
gress and  in  committee,  until  at  last,  in  1821, 
Lambert  was  appointed  "  to  make  astronomical 
observations  by  lunar  occultations  of  fixed  stars, 
solar  eclipses,  or  any  approved  method  adapted 
to  ascertain  the  longitude  of  the  Capitol  from 
Greenwich."  Lambert's  reports  were  made  in 
1822  and  1823,  but  ten  years  passed  before  the 

168 


OCCULTATIONS 

establishment  of  a  formal  Naval  Observatory  un- 
der Goldsborough,  Wilkes,  and  Gilliss.  But  to 
Lambert  belongs  the  honor  of  having  marked 
out  the  first  fundamental  official  meridian  of 
longitude  in  the  United  States. 


169 


MOUNTING    GREAT    TELESCOPES 

THERE  are  many  interesting  practical  things 
about  an  astronomical  observatory  with  which 
the  public  seldom  has  an  opportunity  to  become 
acquainted.  Among  these,  perhaps,  the  details 
connected  with  setting  up  a  great  telescope  take 
first  rank.  The  writer  happened  to  be  present 
at  the  Cape  of  Good  Hope  Observatory  when 
the  photographic  equatorial  telescope  was  being 
mounted,  and  the  operation  of  putting  it  in  posi- 
tion may  be  taken  as  typical  of  similar  processes 
elsewhere.  (See  also  p.  86.) 

In  the  first  place,  it  is  necessary  to  explain 
what  is  meant  by  an  "  equatorial "  telescope. 
One  of  the  chief  difficulties  in  making  ordinary 
observations  arises  from  the  rising  and  setting  of 
the  stars.  They  are  all  apparently  moving  across 
the  face  of  the  sky,  usually  climbing  up  from  the 
eastern  horizon,  only  to  go  down  again  and  set 
in  the  west.  If,  therefore,  we  wish  to  scrutinize 

170 


Forty-Inch  Telescope,  Yerkes   Observatory, 
University   of  Chicago. 


MOUNTING   GREAT   TELESCOPES 

any  given  object  for  a  considerable  time,  we  must 
move  the  telescope  continuously  so  as  to  keep 
pace  with  the  motion  of  the  heavens.  For  this 
purpose,  the  tube  must  be  attached  to  axles,  so 
that  it  can  be  turned  easily  in  any  direction. 
The  equatorial  mounting  is  a  device  that  permits 
the  telescope  to  be  thus  aimed  at  any  part  of  the 
sky,  and  at  the  same  time  facilitates  greatly  the 
operation  of  keeping  it  pointed  correctly  after  a 
star  has  once  been  brought  into  the  field  of  view. 

To  understand  the  equatorial  mounting  it  is 
necessary  to  remember  that  the  rising  and  setting 
motions  of  the  heavenly  bodies  are  apparent  ones 
only,  and  due  in  reality  to  the  turning  of  the 
earth  on  its  own  axis.  As  the  earth  goes  around, 
it  carries  observer,  telescope,  and  observatory  past 
the  stars  fixed  upon  the  distant  sky.  Conse- 
quently, to  keep  a  telescope  pointed  continuously 
at  a  given  star,  it  is  merely  necessary  to  rotate  it 
steadily  backward  upon  a  suitable  axis  just  fast 
enough  to  neutralize  exactly  the  turning  of  our 
earth. 

By  a  suitable  axis   for  this   purpose  we  mean 

one  so  mounted  as  to  be  exactly  parallel  to  the 

171 


MOUNTING   GREAT   TELESCOPES 

earth's  own  axis  of  rotation.  A  little  reflection 
shows  how  simply  such  an  arrangement  will 
work.  All  the  heavenly  bodies  may  be  re- 
garded, for  practical  purposes,  as  excessively  re- 
mote in  comparison  with  the  dimensions  of  our 
earth.  The  entire  planet  shrinks  into  absolute 
insignificance  when  compared  with  the  distances 
of  the  nearest  objects  brought  under  observation 
by  astronomers.  It  follows  that  if  we  have  our 
telescope  attached  to  such  a  rotation-axis  as  we 
have  described,  it  will  be  just  the  same  for  pur- 
poses of  observation  as  though  the  telescope's 
axis  were  not  only  parallel  to  the  earth's  axis,  but 
actually  coincident  with  it.  The  two  axes  may 
be  separated  by  a  distance  equal  to  that  between 
the  earth's  surface  and  its  centre ;  but,  as  we 
have  said,  this  distance  is  insignificant  so  far  as 
our  present  object  is  concerned. 

There  is  another  way  to  arrive  at  the  same 
result.  We  know  that  the  stars  in  rising  and 
setting  all  seem  to  revolve  about  the  pole  star, 
which  itself  seems  to  remain  immovable.  Con- 
sequently, if  we  mount  our  telescope  so  that  it 
can  turn  about  an  axis  pointing  at  the  pole,  we 

172 


MOUNTING   GREAT   TELESCOPES 

shall  be  able  to  neutralize  the  rotation  of  the  stars 
by  simply  turning  the  telescope  about  the  axis  at 
the  proper  speed  and  in  the  right  direction.  As- 
tronomical considerations  teach  us  that  an  axis 
thus  pointing  at  the  pole  will  be  parallel  to  the 
earth's  own  axis.  Thus  we  arrive  at  the  same 
fundamental  principle  for  mounting  an  astronom- 
ical telescope  from  whichever  point  of  view  we 
consider  the  subject. 

Every  large  telescope  is  provided  with  such  an 
axis  of  rotation  ;  and  for  the  reason  stated  it  is 
called  the  "  polar  axis."  The  telescope  itself  is 
then  called  an  "  equatorial."  The  advantage  of 
this  method  of  mounting  is  very  evident.  Since 
we  can  follow  the  stars'  motions  by  turning  the 
telescope  about  one  axis  only,  it  becomes  a  very 
simple  matter  to  accomplish  this  turning  auto- 
matically by  means  of  clock-work. 

The  "  following  "  of  a  star  being  thus  provided 
for  by  the  device  of  a  polar  axis,  it  is,  of  course, 
also  necessary  to  supply  some  other  motion  so  as 
to  enable  us  to  aim  the  tube  at  any  point  in  the 
heavens.  For  it  is  obvious  that  if  it  were  rigidly 
attached  to  the  polar  axis,  we  could,  indeed,  follow 

173 


MOUNTING   GREAT   TELESCOPES 

any  star  that  happened  to  be  in  the  field  of  view, 
but  we  could  not  change  this  field  of  view  at  will 
so  as  to  observe  other  stars  or  planets.  To  ac- 
complish this,  the  telescope  is  attached  to  the 
polar  axis  by  means  of  a  pivot.  By  turning  the 
telescope  around  its  polar  axis,  and  also  on  this 
pivot,  we  can  find  any  object  in  the  heavens  ;  and 
once  found,  we  can  leave  to  the  polar  axis  and  its 
automatic  clock-work  the  task  of  keeping  that 
object  before  the  observer's  eye. 

In  setting  up  the  Cape  of  Good  Hope  instru- 
ment the  astronomers  were  obliged  to  do  a  large 
part  of  the  work  of  adjustment  personally.  Far 
away  from  European  instrument-makers,  the  parts 
of  the  mounting  and  telescope  had  to  be  "  assem- 
bled," or  put  together,  by  the  astronomers  of  the 
Cape  Observatory.  A  heavy  pier  of  brick  and 
masonry  had  been  prepared  in  advance.  Upon 
this  was  placed  a  massive  iron  base,  intended  to 
support  the  superstructure  of  polar  axis  and  tele- 
scope. This  base  rested  on  three  points,  one  of 
which  could  be  screwed  in  and  out,  so  as  to  tilt 
the  whole  affair  a  little  forward  or  backward. 
By  means  of  this  screw  we  effected  the  final  ad- 

174 


MOUNTING   GREAT   TELESCOPES 

justment  of  the  polar  axis  to  exact  parallelism 
with  that  of  the  earth.  Other  screws  were  pro- 
vided with  which  the  base  could  be  twisted  a  little 
horizontally  either  to  the  right  or  left.  Once  set 
up  in  a  position  almost  correct,  it  was  easy  to  per- 
fect the  adjustment  by  the  aid  of  these  screws. 

Afterward  the  tube  and  lenses  were  put  in 
place,  and  the  clock  properly  attached  inside  the 
big  cast-iron  base.  This  clock-work  looked  more 
like  a  piece  of  heavy  machinery  than  a  delicate 
clock  mechanism.  But  it  had  heavy  work  to  do, 
carrying  the  massive  telescope  with  its  weighty 
lenses,  and  needed  to  be  correspondingly  strong. 
It  had  a  driving-weight  of  about  2,000  pounds, 
and  was  so  powerful  that  turning  the  telescope 
affected  it  no  more  than  the  hour-hand  of  an  ordi- 
nary clock  affects  the  mechanism  within  its  case. 

The  final  test  of  the  whole  adjustment  con- 
sisted in  noting  whether  stars  once  brought  into 
the  telescopic  field  of  view  could  be  maintained 
there  automatically  by  means  of  the  clock.  This 
object  having  been  attained  successfully,  the  in- 
strument stood  ready  to  be  used  in  the  routine 
business  of  the  observatory. 

175 


MOUNTING   GREAT   TELESCOPES 

Before  leaving  the  subject  of  telescope-mount- 
ings, we  must  mention  the  giant  instrument  set 
up  at  the  Paris  Exposition  of  1900.  The  project 
of  having  a  Grande  Lunette  had  been  hailed  by 
newspapers  throughout  the  world  and  by  the 
general  public  in  their  customary  excitable  way. 
It  was  tremendously  over-advertised ;  exagger- 
ated notions  of  the  instrument's  powers  were 
spread  abroad  and  eagerly  credited ;  the  moon 
was  to  be  dragged  down,  as  it  were,  from  its 
customary  place  in  the  sky,  so  near  that  we 
should  be  able  almost  to  touch  its  surface.  As 
to  the  planets — free  license  was  given  to  the  jour- 
nalistic imagination,  and  there  was  no  effective 
limitation  to  the  magnificence  of  astronomical 
discovery  practically  within  our  grasp,  beyond 
the  necessity  for  printed  space  demanded  by  sun- 
dry wars,  pestilences,  and  other  mundane  trifles. 

Now,  the  present  writer  is  very  far  from  advo- 
cating a  lessening  of  the  attention  devoted  to  as- 
tronomy. Rather  would  he  magnify  his  office 
than  diminish  it.  But  let  journalistic  astronomy 
be  as  good  an  imitation  of  sober  scientific  truth 

as  can  be  procured  at  space  rates  ;  let  editors  en- 

176 


MOUNTING  GREAT   TELESCOPES 

courage  the  public  to  study  those  things  in  the 
science  that  are  ennobling  and  cultivating  to  the 
mind ;  let  there  be  an  end  to  the  frenzied  effort 
to  fabricate  a  highly  colored  account  of  alleged 
discoveries  of  yesterday,  capable  of  masquerad- 
ing to-day  under  heavy  head-lines  as  News. 

The  manner  in  which  the  big  telescope  came 
to  be  built  is  not  without  interest,  and  shows 
that  enterprise  is  far  from  dead,  even  in  the  old 
countries.  A  stock  company  was  organized — 
we  should  call  it  a  corporation  —  under  the 
name  Societe  de  FOptique.  It  would  appear  that 
shares  were  regularly  put  on  the  market,  and 
that  a  prospectus,  more  or  less  alluring,  was 
widely  distributed.  We  may  say  at  once  that  the 
investing  public  did  not  respond  with  obtrusive 
alacrity  ;  but  at  all  events,  the  promoters'  efforts 
received  sufficient  encouragement  to  enable  them 
to  begin  active  work.  From  the  very  first  a 
vigorous  attempt  was  made  to  utilize  both  the 
resources  of  genuine  science  and  the  devices  of 
quasi-charlatanry.  It  was  announced  that  the 
public  were  to  be  admitted  to  look  through  the 

big  glass   (apparently  at  so  much  an  eye),  and 

177 


MOUNTING   GREAT   TELESCOPES 

many,  doubtless,  expected  that  the  man  in  the 
street  would  be  able  to  make  personal  acquaint- 
ance with  the  man  in  the  moon.  A  telescopic 
image  of  the  sun  was  to  be  projected  on  a  big 
screen,  and  exhibited  to  a  concourse  of  specta- 
tors assembled  in  rising  tiers  of  seats  within  a 
great  amphitheatre.  And  when  clouds  or  other 
circumstances  should  prevent  observing  the  plan- 
ets or  scrutinizing  the  sun,  a  powerful  stereopti- 
con  was  to  be  used.  Artificial  pictures  of  the 
wonders  of  heaven  were  to  be  projected  on  the 
screen,  and  the  public  would  never  be  disap- 
pointed. It  was  arranged  that  skilled  talkers 
should  be  present  to  explain  all  marvels  :  and,  in 
short,  financial  profit  was  to  be  combined  with 
machinery  for  advancing  scientific  discovery. 
Astronomers  the  world  over  were  "  circularized," 
asked  to  become  shareholders,  and,  in  default  of 
that,  to  send  lantern-slides  or  photographs  of 
remarkable  celestial  objects  for  exhibition  in  the 
magic-lantern  part  of  the  show. 

The  project  thus  brought  to  the  attention  of 
scientific  men  three  years  ago  did  not  have  an 
attractive  air.  It  savored  too  much  of  charlatan- 

178 


MOUNTING  GREAT   TELESCOPES 

ism.  But  it  soon  appeared  that  effective  gov- 
ernment sanction  had  been  given  to  the  enter- 
prise ;  and,  above  all,  that  men  of  reputation 
were  allowing  the  use  of  their  names  in  connec- 
tion with  the  affair.  More  important  still,  we 
learned  that  the  actual  construction  had  been 
undertaken  by  Gautier,  of  Paris,  that  finances 
were  favorable,  and  that  real  work  on  parts  of 
the  instrument  was  to  commence  without  delay. 

Gautier  is  a  first-class  instrument-builder ;  he 
has  established  his  reputation  by  constructing  suc- 
cessfully several  telescopes  of  very  large  size,  in- 
cluding the  equatorial  coude  of  the  Paris  Observa- 
tory, a  unique  instrument  of  especial  complexity. 
The  present  writer  believes  that,  if  sufficient  time 
and  money  were  available,  the  Grande  Lunette 
would  stand  a  reasonable  chance  of  success  in 
the  hands  of  such  a  man.  And  by  a  reasonable 
chance,  we  mean  that  there  is  a  large  enough 
probability  of  genuine  scientific  discovery  to 
justify  the  necessary  financial  outlay.  But  the 
project  should  be  divorced  from  its  "  popular " 
features,  and  every  kind  of  advertising  and  char- 
latanism excluded  with  rigor. 

179 


MOUNTING  GREAT   TELESCOPES 

As  planned  originally,  and  actually  constructed, 
the  Grande  Lunette  presents  interesting  peculiari- 
ties, distinguishing  it  from  other  telescopes.  Pre- 
vious instruments  have  been  built  on  the  prin- 
ciple of  universal  mobility.  It  is  possible  to 
move  them  in  all  directions,  and  thus  bring  any 
desired  star  under  observation,  irrespective  of  its 
position  in  the  sky.  But  this  general  mobility 
offers  great  difficulties  in  the  case  of  large  and 
ponderous  telescopes.  Delicacy  of  adjustment  is 
almost  destroyed  when  the  object  to  be  adjusted 
weighs  several  tons.  And  the  excessive  weight 
of  telescopes  is  not  due  to  unavoidably  heavy 
lenses  alone.  It  is  essential  that  the  tube  be 
long;  and  great  length  involves  appreciable 
thickness  of  material,  if  stiffness  and  solidity  are 
to  remain  unsacrifked.  Length  in  the  tube  is 
necessitated  by  certain  peculiar  optical  defects  of 
all  lenses,  into  the  nature  of  which  we  shall  not 
enter  at  present.  The  consequences  of  these 
defects  can  be  rendered  harmless  only  if  the  in- 
strument is  so  arranged  that  the  observer's  eye  is 
far  from  the  other  end  of  the  tube.  The  length 
of  a  good  telescope  should  be  at  least  twelve 

180 


MOUNTING  GREAT   TELESCOPES 

times  the  diameter  of  its  large  lens.  If  the  rela- 
tive length  can  be  still  further  increased,  so 
much  the  better ;  for  then  the  optical  defects 
can  be  further  reduced. 

In  the  case  of  the  Paris  instrument  a  radical 
departure  consists  in  making  the  tube  of  unprec- 
edented length,  197  feet,  with  a  lens  diameter  of 
49^4  inches.  This  great  length,  while  favorable 
optically,  precludes  the  possibility  of  making  the 
instrument  movable  in  the  usual  sense.  In  fact, 
the  entire  tube  is  attached  to  a  fixed  horizontal 
base,  and  no  attempt  is  made  to  change  its  posi- 
tion. Outside  the  big  lens,  and  disconnected 
altogether  from  the  telescope  proper,  is  mounted 
a  smooth  mirror,  so  arranged  that  it  can  be 
turned  in  any  direction,  and  thus  various  parts  of 
the  sky  examined  by  reflection  in  the  telescope. 

While  this  method  unquestionably  has  the  ad- 
vantage of  leaving  the  optician  quite  free  as  to 
how  long  he  will  make  his  tube,  it  suffers  from 
the  compensating  objection  that  a  new  optical 
surface  is  introduced  into  the  combination,  viz., 
the  mirror.  Any  slight  unavoidable  imperfec- 
tion in  the  polishing  of  its  surface  will  infallibly 

181 


MOUNTING  GREAT   TELESCOPES 

be  reproduced  on  a  magnified  scale  in  the  image 
of  a  distant  star  brought  before  the  observer's 
eye. 

But  it  is  not  yet  possible  to  pronounce  defi- 
nitely upon  the  merit  of  this  form  of  instrument, 
since,  as  we  have  said,  the  maker  has  not  been 
given  time  enough  to  try  the  idea  to  the  com- 
plete satisfaction  of  scientific  men.  In  the  early 
part  of  August,  1900,  when  the  informant  of  the 
present  writer  left  Paris,  after  serving  as  a  mem- 
ber of  the  international  jury  for  judging  instru- 
ments of  precision  at  the  Exposition,  the  condi- 
tion of  the  Grande  Lunette  was  as  follows  :  Two 
sets  of  lenses  had  been  contemplated,  one  intend- 
ed for  celestial  photography,  and  the  other  to  be 
used  for  ordinary  visual  observation.  Only  the 
photographic  lenses  had  been  completed,  how- 
ever, and  for  this  reason  the  public  could  not  be 
permitted  to  look  through  the  instrument.  The 
photographic  lenses  were  in  place  in  the  tube,  but 
at  that  time  their  condition  was  such  that,  though 
some  photographs  had  been  obtained,  it  was  not 
thought  advisable  to  submit  them  to  the  jury. 
Consequently,  the  Lunette  did  not  receive  a 

182 


MOUNTING  GREAT   TELESCOPES 

prize.  Since  that  time  various  newspapers  have 
reported  wonderful  results  from  the  telescope ; 
but,  disregarding  the  fusillade  from  the  sensa- 
tional press,  we  may  sum  up  the  present  state  of 
affairs  very  briefly.  Gautier  is  still  experiment- 
ing; and,  given  sufficient  time  and  money,  he 
may  succeed  in  producing  what  astronomers 
hope  for — an  instrument  capable  of  advancing 
our  knowledge,  even  if  that  advance  be  only  a 
small  one. 


183 


THE    ASTRONOMER'S    POLE 

THE  pole  of  the  frozen  North  is  not  the  only 
pole  sought  with  determined  effort  by  more  than 
one  generation  of  scientific  men.  Up  in  the  sky 
astronomers  have  another  pole  which  they  are 
following  up  just  as  vigorously  as  ever  Arctic  ex- 
plorer struggled  toward  the  difficult  goal  of  his  ter- 
restrial journeying.  The  celestial  pole  is,  indeed, 
a  fundamentally  important  thing  in  astronomical 
science,  and  the  determination  of  its  exact  position 
upon  the  sky  has  always  engaged  the  closest 
attention  of  astronomers.  Quite  recently  new 
methods  of  research  have  been  brought  to  bear, 
promising  a  degree  of  success  not  hitherto  at- 
tained in  the  astronomers*  pursuit  of  their  pole. 

In  the  first  place,  we  must  explain  what  is 
meant  by  the  celestial  pole.  We  have  already 
mentioned  the  poles  of  the  earth  (p.  136).  Our 
planet  turns  once  daily  upon  an  axis  passing 
through  its  centre,  and  it  is  this  rotation  that 

184 


THE  ASTRONOMER'S   POLE 

causes  all  the  so-called  diurnal  phenomena  of  the 
heavens.  Rising  and  setting  of  sun,  moon,  and 
stars  are  simply  results  of  this  turning  of  the 
earth.  Heavenly  bodies  do  not  really  rise ;  it  is 
merely  the  man  on  the  earth  who  is  turned  round 
on  an  axis  until  he  is  brought  into  a  position  from 
which  he  can  see  them.  The  terrestrial  poles  are 
those  two  points  on  the  earth's  surface  where  it 
is  pierced  by  the  rotation  axis  of  the  planet. 
Now  we  can,  if  we  choose,  imagine  this  axis 
lengthened  out  indefinitely,  further  and  further, 
until  at  last  it  reaches  the  great  round  vault  of 
the  sky.  Here  it  will  again  pierce  out  two  polar 
points ;  and  these  are  the  celestial  poles. 

The  whole  thing  is  thus  quite  easy  to  un- 
derstand. On  the  sky  the  poles  are  marked  by 
the  prolongation  of  the  earth's  axis,  just  as  on 
the  earth  the  poles  are  marked  by  the  axis  itself. 
And  this  explains  at  once  why  the  stars  seem 
nightly  to  revolve  about  the  pole.  If  the  ob- 
server is  being  turned  round  the  earth's  axis,  of 
course  it  will  appear  to  him  as  if  the  stars  were 
rotating  around  the  same  axis  in  the  opposite 
direction,  just  as  houses  and  fields  seem  to  fly 

185 


THE  ASTRONOMER'S   POLE 

past  a  person  sitting  in  a  railway  train,  unless  he 
stops  to  remember  that  it  is  really  himself  who  is 
in  motion,  and  not  the  trees  and  houses. 

The  existence  of  such  a  centre  of  daily  motions 
among  the  stars  once  recognized,  it  becomes  of 
interest  to  ascertain  whether  the  centre  itself  al- 
ways retains  precisely  the  same  position  in  the 
sky.  It  was  discovered  as  early  as  the  time  of 
Hipparchus  (p.  39)  that  such  is  not  the  case, 
and  that  the  celestial  pole  is  subject  to  a  slow 
motion  among  the  stars  on  the  sky.  If  a  given 
star  were  to-day  situated  exactly  at  the  pole,  it 
would  no  longer  be  there  after  the  lapse  of  a 
year's  time  ;  for  the  pole  would  have  moved  away 
from  it. 

This  motion  of  the  pole  is  called  precession. 
It  means  that  certain  forces  are  continually  at 
work,  compelling  the  earth's  axis  to  change  its 
position,  so  that  the  prolongation  of  that  axis 
must  pierce  the  sky  at  a  point  which  moves  as 
time  goes  on.  These  forces  are  produced  by  the 
gravitational  attractions  of  the  sun,  moon,  and 
planets  upon  the  matter  composing  our  earth. 
If  the  earth  were  perfectly  spherical  in  shape, 

1 86 


THE   ASTRONOMER'S   POLE 

the  attractions  of  the  other  heavenly  bodies 
would  not  affect  the  direction  of  the  earth's  rota- 
tion-axis in  the  least.  But  the  earth  is  not  quite 
globular  in  form ;  it  is  flattened  a  little  at  the 
poles  and  bulges  out  somewhat  at  the  equator. 
(Seep.  135.) 

This  protuberant  matter  near  the  equator  gives 
the  other  bodies  in  the  solar  system  an  oppor- 
tunity to  disturb  the  earth's  rotation.  The  gen- 
eral effect  of  all  these  attractions  is  to  make  the 
celestial  pole  move  upon  the  sky  in  a  circle  hav- 
ing a  radius  of  about  23  j£  degrees  ;  and  it  re- 
quires 25,800  years  to  complete  a  circuit  of  this 
precessional  cycle.  One  of  the  most  striking  con- 
sequences of  this  motion  will  be  the  change  of  the 
polar  star.  Just  at  present  the  bright  star  Polaris 
in  the  constellation  of  the  Little  Bear  is  very 
close  to  the  pole.  But  after  the  lapse  of  sufficient 
ages  the  first-magnitude  star  Vega  of  the  constella- 
tion Lyra  will  in  its  turn  become  Guardian  of  the 
Pole. 

It  must  not  be  supposed,  however,  that  the 
motion  of  the  pole  proceeds  quite  uniformly,  and 
in  an  exact  circle ;  the  varying  positions  of  the 

187 


THE  ASTRONOMER'S   POLE 

heavenly  bodies  whose  attractions  cause  the  phe- 
nomena in  question  are  such  as  to  produce  ap- 
preciable divergencies  from  exact  circular  motion. 
Sometimes  the  pole  deviates  a  little  to  one  side 
of  the  precessional  circle,  and  sometimes  it  de- 
viates on  the  other  side.  The  final  result  is  a 
sort  of  wavy  line,  half  on  one  side  and  half  on 
the  other  of  an  average  circular  curve.  It  takes 
only  nineteen  years  to  complete  one  of  these  little 
waves  of  polar  motion,  so  that  in  the  whole  pre- 
cessional cycle  of  25,800  years  there  are  about 
1,400  indentations.  This  disturbance  of  the  polar 
motion  is  called  by  astronomers  nutation. 

The  first  step  in  a  study  of  polar  motion  is  to 
devise  a  method  of  finding  just  where  the  pole  is 
on  any  given  date.  If  the  astronomer  can  ascer- 
tain by  observational  processes  just  where  the 
pole  is  among  the  stars  at  any  moment,  and  can 
repeat  his  observations  year  after  year  and  genera- 
tion after  generation,  he  will  possess  in  time  a 
complete  chart  of  a  small  portion  at  least  of  the 
celestial  pole's  vast  orbit.  From  this  he  can  ob- 
tain necessary  data  for  a  study  of  the  mathematical 
theory  of  attractions,  and  thus,  perhaps,  arrive  at 

188 


THE   ASTRONOMER'S   POLE 

an  explanation  of  the  fundamental  laws  governing 
the  universe  in  which  we  live. 

The  instrument  which  has  been  used  most  ex- 
tensively for  the  study  of  these  problems  is  the 
transit  (p.  118)  or  the  "meridian  circle."  This 
latter  consists  of  a  telescope  firmly  attached  to  a 
metallic  axis  about  which  it  can  turn.  The  axis 
itself  rests  on  massive  stone  supports,  and  is  so 
placed  that  it  points  as  nearly  as  possible  in  an 
east-and-west  direction.  Consequently,  when  the 
telescope  is  turned  about  its  axis,  it  will  trace  out 
on  the  sky  a  great  circle  (the  meridian)  which 
passes  through  the  north  and  south  points  of  the 
horizon  and  the  point  directly  overhead.  The 
instrument  has  also  a  metallic  circle  very  firmly 
fastened  to  the  telescope  and  its  axis.  Let  into 
the  surface  of  this  circle  is  a  silver  disk  upon 
which  are  engraved  a  series  of  lines  or  gradua- 
tions by  means  of  which  it  is  impossible  to  meas- 
ure angles. 

Observers  with  the  meridian  circle  begin  by 
noting  the  exact  instant  when  any  given  star 
passes  the  centre  of  the  field  of  view  of  the  tele- 
scope. This  centre  is  marked  with  a  cross  made 

189 


THE  ASTRONOMER'S   POLE 

by  fastening  into  the  focus  some  pieces  of  ordi- 
nary spider's  web,  which  give  a  well-marked,  deli- 
cate set  of  lines,  even  under  the  magnifying  power 
of  the  telescope's  eye-piece.  In  addition  to  thus 
noting  the  time  when  the  star  crosses  the  field 
of  the  telescope,  the  astronomer  can  measure  by 
means  of  the  circle,  how  high  up  it  was  in  the  sky 
at  the  instant  when  it  was  thus  observed. 

If  the  telescope  of  the  meridian  circle  be  turned 
toward  the  north,  and  we  observe  stars  close  to 
the  pole,  it  is  possible  to  make  two  different  ob- 
servations of  the  same  star.  For  the  close  polar 
stars  revolve  in  such  small  circles  around  the  pole 
of  the  heavens  that  we  can  observe  them  when 
they  are  on  the  meridian  either  above  the  pole  or 
below  it.  Double  observations  of  this  class  en- 
able us  to  obtain  the  elevation  of  the  pole  above 
the  horizon,  and  to  fix  its  position  with  respect 
to  the  stars. 

Now,  there  is  one  very  serious  objection  to 
this  method.  In  order  to  secure  the  two  neces- 
sary observations  of  the  same  star,  it  is  essential 
to  be  stationed  at  the  instrument  at  two  moments 
of  time  separated  by  exactly  twelve  hours ;  and  if 

190 


THE  ASTRONOMER'S   POLE 

one  of  the  observations  occurs  in  the  night,  the 
other  corresponding  observation  will  occur  in 
daylight. 

It  is  a  fact  not  generally  known  that  the 
brighter  stars  can  be  seen  with  a  telescope,  even 
when  the  sun  is  quite  high  above  the  horizon. 
Unfortunately,  however,  there  is  only  one  star 
close  to  the  pole  which  is  bright  enough  to  be 
thus  observed  in  daylight — the  polar  star  already 
mentioned  under  the  name  Polaris.  The  fact 
that  we  are  thus  limited  to  observations  of  a  single 
star  has  made  it  difficult  even  for  generations  of 
astronomers  to  accumulate  with  the  meridian  cir- 
cle a  very  large  quantity  of  observational  material 
suitable  for  the  solution  of  our  problem. 

The  new  method  of  observation  to  which  we 
have  referred  above  consists  in  an  application  of 
photography  to  the  polar  problem.  If  we  aim 
at  the  pole  a  powerful  photographic  telescope, 
and  expose  a  photographic  plate  throughout  the 
entire  night,  we  shall  find  that  all  stars  coming 
within  the  range  of  the  plate  will  mark  out  little 
circles  or  "  trails  "  upon  the  developed  negative. 
It  is  evident  that  as  the  stars  revolve  about  the 

191 


THE  ASTRONOMER'S   POLE 

pole  on  the  sky,  tracing  out  their  daily  circular 
orbits,  these  same  little  circles  must  be  repro- 
duced faithfully  upon  the  photographic  plate. 
The  only  condition  is  that  the  stars  shall  be 
bright  enough  to  make  their  light  affect  the  sen- 
sitive gelatine  surface. 

But  even  if  observations  of  this  kind  are  con- 
tinued throughout  all  the  hours  of  darkness,  we 
do  not  obtain  complete  circles,  but  only  those 
portions  of  circles  traced  out  on  the  sky  between 
sunset  and  sunrise.  If  the  night  is  twelve  hours 
in  length,  we  get  half-circles  on  the  plate ;  if  it  is 
eighteen  hours  long,  we  get  circles  that  lack  only 
one-quarter  of  being  complete.  In  other  words, 
we  get  a  series  of  circular  arcs,  one  corresponding 
to  each  close  polar  star.  There  are  no  fewer  than 
sixteen  stars  near  enough  to  the  pole  to  come 
within  the  range  of  a  photographic  plate,  and 
bright  enough  to  cause  measurable  impressions 
upon  the  sensitive  surface.  The  fact  that  the 
circular  arcs  are  not  complete  circles  does  not  in 
the  least  prevent  our  using  them  for  ascertaining 
the  position  of  their  common  centre ;  and  that 
centre  is  the  pole.  Moreover,  as  the  arcs  are 

192 


THE   ASTRONOMER'S   POLE 

distributed  at  all  sorts  of  distances  from  the  pole 
and  in  all  directions,  corresponding  to  the  acci- 
dental positions  of  the  stars  on  the  sky,  we  have 
a  state  of  affairs  extremely  favorable  to  the  ac- 
curate determination  of  the  pole's  place  among 
the  stars  by  means  of  microscopic  measurements 
of  the  plate. 

It  will  be  perceived  that  this  method  is  ex- 
tremely simple,  and,  therefore,  likely  to  be  suc- 
cessful ;  though  its  simplicity  is  slightly  impaired 
by  the  phenomenon  known  to  astronomers  as 
"  atmospheric  refraction."  The  rays  of  light 
coming  down  to  our  telescopes  from  a  distant 
star  must  pass  through  the  earth's  atmosphere 
before  they  reach  us ;  and  in  passing  thus  from 
the  nothingness  of  outer  space  into  the  denser 
material  of  the  air,  they  are  bent  out  of  their 
straight  course.  The  phenomenon  is  analogous 
to  what  we  see  when  we  push  a  stick  down 
through  the  surface  of  still  water ;  we  notice  that 
the  stick  appears  to  be  bent  at  the  point  where  it 
pierces  the  surface  of  the  water ;  and  in  just  the 
same  way  the  rays  of  light  are  bent  when  they 
pierce  into  the  air.  Fortunately,  the  mathemati- 

193 


THE  ASTRONOMER'S   POLE 

cal  theory  of  this  atmospheric  bending  of  light  is 
well  understood,  so  that  it  is  possible  to  remove 
the  effects  of  refraction  from  our  results  by  a 
process  of  calculation.  In  other  words,  we  can 
transform  our  photographic  measures  into  what 
they  would  have  been  if  no  such  thing  as  at- 
mospheric refraction  existed.  This  having  been 
done,  all  the  arcs  on  the  plate  should  be  exactly 
circular,  and  their  common  centre  should  be  the 
position  of  the  pole  among  the  stars  on  the  night 
when  the  photograph  was  made. 

It  is  possible  to  facilitate  the  removal  of  re- 
fraction effects  very  much  by  placing  our  photo- 
graphic telescope  at  some  point  on  the  earth  situ- 
ated in  a  very  high  latitude.  The  elevation  of 
the  pole  above  the  horizon  is  greatest  in  high 
latitudes.  Indeed,  if  Arctic  voyagers  could  ever 
reach  the  pole  of  the  earth  they  would  see  the 
pole  of  the  heavens  directly  overhead.  Now, 
the  higher  up  the  pole  is  in  the  sky,  the  less  will 
be  the  effects  of  atmospheric  refraction  ;  for  the 
rays  of  light  will  then  strike  the  atmosphere  in  a 
direction  nearly  perpendicular  to  its  surface,  which 
is  favorable  to  diminishing  the  amount  of  bending. 

i94 


THE   ASTRONOMER'S   POLE 

There  is  also  another  very  important  ad- 
vantage in  placing  the  telescope  in  a  high  lati- 
tude ;  in  the  middle  of  winter  the  nights  are  very 
long  there  ;  if  we  could  get  within  the  Arctic. 
Circle  itself,  there  would  be  nights  when  the 
hours  of  darkness  would  number  twenty-four, 
and  we  could  substitute  complete  circles  for  our 
broken  arcs.  This  would,  indeed,  be  most  fa- 
vorable from  the  astronomical  point  of  view;  but 
the  essential  condition  of  convenience  for  the  ob- 
server renders  an  expedition  to  the  frozen  Arctic 
regions  unadvisable. 

But  it  is  at  least  possible  to  place  the  tele- 
scope as  far  north  as  is  consistent  with  retaining 
it  within  the  sphere  of  civilized  influences.  We 
can  put  it  in  that  one  of  existing  observatories  on 
the  earth  which  has  the  highest  latitude ;  and 
this  is  the  observatory  of  Helsingfors,  in  Fin- 
land, which  belongs  to  a  great  university,  is 
manned  by  competent  astronomers,  and  has  a 
latitude  greater  than  60  degrees. 

Dr.  Anders  Donner,  Director  of  the  Helsing- 
fors Observatory,  has  at  its  disposal  a  fine  photo- 
graphic telescope,  and  with  this  some  prelimi- 

195 


THE   ASTRONOMER'S   POLE 

nary  experimental  "trail"  photographs  were  made 
in  1895.  These  photographs  were  sent  to  Co- 
lumbia University,  New  York,  and  were  there 
measured  under  the  writer's  direction.  Calcula- 
tions based  on  these  measures  indicate  that  the 
method  is  promising  in  a  very  high  degree ;  and 
it  was,  therefore,  decided  to  construct  a  special 
photographic  telescope  better  adapted  to  the  par- 
ticular needs  of  the  problem  in  hand. 

The  desirability  of  a  new  telescope  arises  from 
the  fact  that  we  wish  the  instrument  to  remain  ab- 
solutely unmoved  during  all  the  successive  hours 
of  the  photographic  exposure.  It  is  clear  that 
if  the  telescope  moves  while  the  stars  are  tracing 
out  their  little  trails  on  the  plate,  the  circularity 
of  the  curves  will  be  disturbed.  Now,  ordinary 
astronomical  telescopes  are  always  mounted  upon 
very  stable  foundations,  well  adapted  to  making 
the  telescope  stand  still ;  but  the  polar  telescope 
which  we  wish  to  use  in  a  research  fundamental 
to  the  entire  science  of  astronomy  ought  to  pos- 
sess immobility  and  stability  of  an  order  higher 
than  that  required  for  ordinary  astronomical  pur- 
poses. 

196 


THE   ASTRONOMER'S   POLE 

It  is  a  remarkable  peculiarity  of  the  instru- 
ment needed  for  the  new  trail  photographs  that 
it  is  never  moved  at  all.  Once  pointed  at  the 
pole,  it  is  ready  for  all  the  observations  of  suc- 
cessive generations  of  astronomers.  It  should 
have  no  machinery,  no  pivots,  axes,  circles, 
clocks,  or  other  paraphernalia  of  the  usual  equa- 
torial telescope.  All  we  want  is  a  very  heavy 
stone  pier,  with  a  telescope  tube  firmly  fastened 
to  it  throughout  its  entire  length.  The  top  of 
the  pier  having  been  cut  to  the  proper  angle  of 
the  pole's  elevation,  and  the  telescope  cemented 
down,  everything  is  complete  from  the  instru- 
mental side ;  and  just  such  an  instrument  as 
this  is  now  ready  for  use  at  Helsingfors. 

The  late  Miss  Catharine  Wolfe  Bruce,  of 
New  York,  was  much  interested  in  the  writer's 
proposed  polar  investigations,  and  in  October, 
1898,  she  contributed  funds  for  the  construction 
of  the  new  telescope,  and  the  Russian  authori- 
ties have  generously  undertaken  the  expense  of  a 
building  to  hold  the  instrument  and  the  granite 
foundation  upon  which  it  rests.  Photographs 

are  now  being  secured  with  the  new  instrument, 

197 


THE  ASTRONOMER'S   POLE 

and  they  will  be  sent  to  Columbia  University, 
New  York,  for  measurement  and  discussion.  It 
is  hoped  that  they  will  carry  out  the  promise  of 
the  preliminary  photographs  made  in  1895 
a  less  suitable  telescope  of  the  ordinary  form. 


198 


THE    MOON    HOAX 

THE  public  attitude  toward  matters  scientific 
is  one  of  the  mysteries  of  our  time.  It  can  be 
described  best  by  the  single  word,  Credulity ; 
simple,  absolute  credulity.  Perfect  confidence  is 
the  most  remarkable  characteristic  of  this  unbe- 
lieving age.  No  charlatan,  necromancer,  or  as- 
trologer of  three  centuries  ago  commanded  more 
respectful  attention  than  does  his  successor  of 
to-day. 

Any  person  can  be  a  scientific  authority;  he 
has  but  to  call  himself  by  that  title,  and  every- 
one will  give  him  respectful  attention.  Numer- 
ous instances  can  be  adduced  from  the  experience 
of  very  recent  years  to  show  how  true  are  these 
remarks.  We  have  had  the  Keeley  motor  and 
the  liquid-air  power  schemes  for  making  some- 
thing out  of  nothing.  Extracting  gold  from  sea- 
water  has  been  duly  heralded  on  scientific  author- 
ity as  an  easy  source  of  fabulous  wealth  for  the 

199 


THE   MOON   HOAX 

million.  Hard-headed  business  men  not  only 
believe  in  such  things,  but  actually  invest  in  them 
their  most  valued  possession,  capital.  Venders 
of  nostrums  and  proprietary  medicines  acquire 
wealth  as  if  by  magic,  though  it  needs  but  a  mo- 
ment's reflection  to  realize  that  these  persons  can- 
not possibly  be  in  possession  of  any  drugs,  or 
secret  methods  of  compounding  drugs,  that  are 
unknown  to  scientific  chemists. 

If  the  world,  then,  will  persistently  intrust  its 
health  and  wealth  into  the  safe-keeping  of  charla- 
tans, what  can  we  expect  when  things  supposedly 
of  far  less  value  are  at  stake  ?  The  famous  Moon 
Hoax,  as  we  now  call  it,  is  truly  a  classic  piece  of 
lying.  Though  it  dates  from  as  long  ago  as 
1835,  it  nas  never  had  an  equal  as  a  piece  of 
"  modern  "  journalism.  Nothing  could  be  more 
useful  than  to  recall  it  to  public  attention  at  least 
once  every  decade;  for  it  teaches  an  important 
lesson  that  needs  to  be  iterated  again  and  again. 

On  November  13,  1833,  Sir  John  Herschel 
embarked  on  the  Mountstuart  Elphinstone,  bound 
for  the  Cape  of  Good  Hope.  He  took  with 
him  a  collection  of  astronomical  instruments, 

200 


THE  MOON  HOAX 

with  which  he  intended  to  study  the  heavens 
of  the  southern  hemisphere,  and  thus  extend 
his  father's  great  work  to  the  south  polar  stars. 
An  earnest  student  of  astronomy,  he  asked  no 
better  than  to  be  left  in  peace  to  seek  the  truth 
in  his  own  fashion.  Little  did  he  think  that  his 
expedition  would  be  made  the  basis  for  a  fabrica- 
tion of  alleged  astronomical  discoveries  destined 
to  startle  a  hemisphere.  Yet  that  is  precisely 
what  happened.  Some  time  about  the  middle  of 
the  year  1835  tne  New  York  Sun  began  the  pub- 
lication of  certain  articles,  purporting  to  give  an 
account  of  "  Great  Astronomical  Discoveries, 
lately  made  by  Sir  John  Herschel  at  the  Cape  of 
Good  Hope."  It  was  alleged  that  these  articles 
were  taken  from  a  supplement  to  the  Edinburgh 
Journal  of  Science;  yet  there  is  no  doubt  that 
they  were  manufactured  entirely  in  the  United 
States,  and  probably  in  New  York. 

The  hoax  begins  at  once  in  a  grandiloquent 
style,  calculated  to  attract  popular  attention,  and 
well  fitted  to  the  marvels  about  to  be  related. 
Here  is  an  introductory  remark,  as  a  specimen  : 
"  It  has  been  poetically  said  that  the  stars  of 


THE  MOON   HOAX 

heaven  are  the  hereditary  regalia  of  man  as  the 
intellectual  sovereign  of  the  animal  creation.  He 
may  now  fold  the  zodiac  around  him  with  a 
loftier  consciousness  of  his  mental  supremacy.'' 
Then  follows  a  circumstantial  and  highly  plausible 
account  of  the  manner  in  which  early  and  exclu- 
sive information  was  obtained  from  the  Cape. 
This  was,  of  course,  important  in  order  to  make 
people  believe  in  the  genuineness  of  the  whole ; 
but  we  pass  at  once  to  the  more  interesting  ac- 
count of  Herschel's  supposed  instrument. 

Nothing  could  be  more  skilful  than  the  way  in 
which  an  air  of  truth  is  cast  over  the  coming  ac- 
count of  marvellous  discoveries  by  explaining  in 
detail  the  construction  of  the  imaginary  Her- 
schelian  instrument.  Sir  John  is  supposed  to 
have  had  an  interesting  conversation  in  England 
"with  Sir  David  Brewster,  upon  the  merits  of 
some  ingenious  suggestion  by  the  latter,  in  his  ar- 
ticle on  optics  in  the  Edinburgh  Encyclopaedia 
(p.  644),  for  improvements  in  the  Newtonian 
reflectors."  The  exact  reference  to  a  particular 
page  is  here  quite  delightful.  After  some  further 
talk,  "  the  conversation  became  directed  to  that 


202 


THE   MOON    HOAX 

all-invincible  enemy,  the  paucity  of  light  in  pow- 
erful magnifiers.  After  a  few  moments'  silent 
thought,  Sir  John  diffidently  inquired  whether  it 
would  not  be  possible  to  effect  a  transfusion  of 
artificial  light  through  the  focal  object  of  vision ! 
Sir  David,  somewhat  startled  at  the  originality  of 
the  idea,  paused  awhile,  and  then  hesitatingly  re- 
ferred to  the  refrangibility  of  rays,  and  the  an- 
gle of  incidence.  .  .  .  Sir  John  continued, 
c  Why  cannot  the  illuminated  microscope,  say 
the  hydro-oxygen,  be  applied  to  render  distinct, 
and,  if  necessary,  even  to  magnify  the  focal  ob- 
ject ? '  Sir  David  sprang  from  his  chair  in  an 
ecstasy  of  conviction,  and  leaping  half-way  to  the 
ceiling,  exclaimed,  cThou  art  the  man/  '  This 
absurd  imaginary  conversation  contains  nothing 
but  an  assemblage  of  optical  jargon,  put  together 
without  the  slightest  intention  of  conveying  any 
intelligible  meaning  to  scientific  people.  Yet  it 
was  well  adapted  to  deceive  the  public ;  and  we 
should  not  be  surprised  if  it  would  be  credited 
by  many  newspaper  readers  to-day. 

The  authors  go  on  to  explain  how  money  was 

raised  to  build  the  new  instrument,  and  then  de- 

203 


THE   MOON   HOAX 

scribe  Herschel's  embarkation  and  the  difficulties 
connected  with  transporting  his  gigantic  ma- 
chines to  the  place  selected  for  the  observing 
station.  "  Sir  John  accomplished  the  ascent  to 
the  plains  by  means  of  two  relief  teams  of  oxen, 
of  eighteen  each,  in  about  four  days,  and,  aided 
by  several  companies  of  Dutch  boors  [j/V],  pro- 
ceeded at  once  to  the  erecting  of  his  gigantic 
fabric."  The  place  really  selected  by  Herschel 
cannot  be  described  better  than  in  his  own 
words,  contained  in  a  genuine  letter  dated  Janu- 
ary 21,  1835:  "A  perfect  paradise  in  rich  and 
magnificent  mountain  scenery,  sheltered  from  all 
winds.  ...  I  must  reserve  for  my  next  all 
description  of  the  gorgeous  display  of  flowers 
which  adorn  this  splendid  country,  as  well  as 
the  astonishing  brilliancy  of  the  constellations." 
The  author  of  the  hoax  could  have  had  no 
knowledge  of  Herschel's  real  location,  as  de- 
scribed in  this  letter. 

The  present  writer  can  bear  witness  to  the 
correctness  of  Herschel's  words.  Feldhausen 
is  truly  an  ideal  secluded  spot  for  astronomical 
study.  A  small  obelisk  under  the  sheer  cliff  of 

204 


THE   MOON   HOAX 

far-famed  Table  Mountain  now  marks  the  site 
of  the  great  reflecting  telescope.  Here  Herschel 
carried  on  his  scrutiny  of  the  Southern  skies. 
He  observed  1,202  double  stars  and  1,708 
nebulae  and  clusters,  of  which  only  439  were  al- 
ready known.  He  studied  the  famous  Magel- 
lanic  clouds,  and  made  the  first  careful  drawings 
of  the  "  keyhole "  nebula  in  the  constellation 
Argo. 

Very  recent  researches  of  the  present  royal 
astronomer  at  the  Cape  have  shown  that  changes 
of  import  have  certainly  taken  place  in  this 
nebula  since  Herschel's  time,  when  a  sudden 
blazing  up  of  the  wonderful  star  Eta  Argus 
was  seen  within  the  nebula.  This  object  has, 
perhaps,  undergone  more  remarkable  changes  of 
light  than  any  other  star  in  the  heavens.  It  is 
as  though  there  were  some  vast  conflagration  at 
work,  now  blazing  into  incandescence,  and  again 
sinking  almost  into  invisibility.  In  1843  Ma- 
clear  estimated  the  brilliancy  of  Eta  to  be  about 
equal  to  that  of  Sirius,  the  brightest  star  in  the 
whole  sky.  Later  it  diminished  in  light,  and 

cannot  be  seen  to-day  with  the  naked  eye,  though 

205 


THE  MOON   HOAX 

the  latest  telescopic  observations  indicate  that  it 
is  again  beginning  to  brighten. 

Such  was  Herschel's  quiet  study  of  his  beloved 
science,  in  glaring  contrast  to  the  supposed  dis- 
coveries of  the  "  Hoax."  Here  are  a  few  things 
alleged  to  have  been  seen  on  the  moon.  The 
first  time  the  instrument  was  turned  upon  our  sat- 
ellite "  the  field  of  view  was  covered  throughout 
its  entire  area  with  a  beautifully  distinct  arid  even 
vivid  representation  of  basaltic  rock.'*  There 
were  forests,  too,  and  water,  "  fairer  shores  never 
angels  coasted  on  a  tour  of  pleasure.  A  beach  of 
brilliant  white  sand,  girt  with  wild  castellated 
rocks,  apparently  of  green  marble." 

There  was  animal  life  as  well ;  "  we  beheld 
continuous  herds  of  brown  quadrupeds,  having 
all  the  external  characteristics  of  the  bison, 
but  more  diminutive  than  any  species  of  the 
bos  genus  in  our  natural  history."  There  was 
a  kind  of  beaver,  that  "  carries  its  young  in 
its  arms  like  a  human  being,"  and  lives  in  huts. 
"  From  the  appearance  of  smoke  in  nearly  all  of 
them,  there  is  no  doubt  of  its  (the  beaver's)  be- 
ing acquainted  with  the  use  of  fire."  Finally,  as 

206 


THE   MOON '  HOAX 

was,  of  course,  unavoidable,  human  creatures 
were  discovered.  "Whilst  gazing  in  a  perspec- 
tive of  about  half  a  mile,  we  were  thrilled  with 
astonishment  to  perceive  four  successive  flocks  of 
large-winged  creatures,  wholly  unlike  any  kind 
of  birds,  descend  with  a  slow,  even  motion  from 
the  cliffs  on  the  western  side,  and  alight  upon 
the  plain.  .  .  .  Certainly  they  were  like  hu- 
man beings,  and  their  attitude  in  walking  was 
both  erect  and  dignified." 

We  have  not  space  to  give  more  extended  ex- 
tracts from  the  hoax,  but  we  think  the  above 
specimens  will  show  how  deceptive  the  whole 
thing  was.  The  rare  reprint  from  which  we  have 
extracted  our  quotations  contains  also  some  inter- 
esting "  Opinions  of  the  American  Press  Respect- 
ing the  Foregoing  Discovery."  The  Daily  Ad- 
vertiser said :  "  No  article,  we  believe,  has  ap- 
peared for  years,  that  will  command  so  general  a 
perusal  and  publication.  Sir  John  has  added  a 
stock  of  knowledge  to  the  present  age  that  will 
immortalize  his  name  and  place  it  high  on  the 
page  of  science."  The  Mercantile  Advertiser 

said  :    "  Discoveries   in   the    Moon.  — We    com- 

207 


THE  MOON   HOAX 

mence  to-day  the  publication  of  an  interesting 
article  which  is  stated  to  have  been  copied  from 
the  Edinburgh  Journal  of  Science,  and  which  made 
its  first  appearance  here  in  a  contemporary  journal 
of  this  city.  It  appears  to  carry  intrinsic  evidence 
of  being  an  authentic  document."  Many  other 
similar  extracts  are  given.  The  New  York  Even-* 
ing  Post  did  not  fall  into  the  trap.  The  Evening 
Post's  remarks  were  as  follows:  "It  is  quite 
proper  that  the  Sun  should  be  the  means  of  shed- 
ding so  much  light  on  the  Moon.  That  there 
should  be  winged  people  in  the  moon  does  not 
strike  us  as  more  wonderful  than  the  existence  of 
such  a  race  of  beings  on  the  earth  ;  and  that  there 
does  or  did  exist  such  a  race  rests  on  the  evidence 
of  that  most  veracious  of  voyagers  and  circum- 
stantial of  chroniclers.  Peter  Wilkins,  whose  cele- 
brated work  not  only  gives  an  account  of  the 
general  appearance  and  habits  of  a  most  interest- 
ing tribe  of  flying  Indians,  but  also  of  all  those 
more  delicate  and  engaging  traits  which  the  author 
was  enabled  to  discover  by  reason  of  the  conjugal 
relations  he  entered  into  with  one  of  the  females 
of  the  winged  tribe." 

208 


THE   MOON   HOAX 

We  shall  limit  our  extracts  from  the  contem- 
porary press  to  the  few  quotations  here  given, 
hoping  that  enough  has  been  said  to  direct  atten- 
tion once  more  to  that  important  subject,  the 
Possibility  of  Being  Deceived. 


209 


THE    SUN'S    DESTINATION 

THREE  generations  of  men  have  come  and 
gone  since  the  Marquis  de  Laplace  stood  before 
the  Academy  of  France  and  gave  his  demonstra- 
tion of  the  permanent  stability  of  our  solar  sys- 
tem. There  was  one  significant  fault  in  New- 
ton's superbly  simple  conception  of  an  eternal 
law  governing  the  world  in  which  we  live.  The 
labors  of  mathematicians  following  him  had 
shown  that  the  planets  must  trace  out  paths  in 
space  whose  form  could  be  determined  in  ad- 
vance with  unerring  certainty  by  the  aid  of 
Newton's  law  of  gravitation.  But  they  proved 
just  as  conclusively  that  these  planetary  orbits, 
as  they  are  called,  could  not  maintain  indefinitely 
the  same  shapes  or  positions.  Slow  indeed  might 
be  the  changes  they  were  destined  to  undergo  ; 
slow,  but  sure,  with  that  sureness  belonging  to 
celestial  science  alone.  And  so  men  asked : 
Has  this  magnificent  solar  system  been  built 


2IO 


THE  SUN'S  DESTINATION 

upon  a  scale  so  grand,  been  put  in  operation  sub- 
ject to  a  law  sublime  in  its  very  simplicity,  only 
to  change  and  change  until  at  length  it  shall  lose 
every  semblance  of  its  former  self,  and  end,  per- 
haps, in  chaos  or  extinction  ? 

Laplace  was  able  to  answer  confidently,  "  No." 
Nor  was  his  answer  couched  in  the  enthusiastic 
language  of  unbalanced  theorists  who  work  by 
the  aid  of  imagination  alone.  Based  upon  the 
irrefragable  logic  of  correct  mathematical  reason- 
ing, and  clad  in  the  sober  garb  of  mathematical 
formulae,  his  results  carried  conviction  to  men  of 
science  the  world  over.  So  was  it  demonstrated 
that  changes  in  our  solar  system  are  surely  at 
work,  and  shall  continue  for  nearly  countless 
ages ;  yet  just  as  surely  will  they  be  reversed  at 
last,  and  the  system  will  tend  to  return  again  to 
its  original  form  and  condition.  The  objection 
that  the  Newtonian  law  meant  ultimate  dissolu- 
tion of  the  world  was  thus  destroyed  by  Laplace. 
From  that  day  forward  the  law  of  gravitation 
has  been  accepted  as  holding  sway  over  all  phe- 
nomena visible  within  our  planetary  world. 

The  intricacies  of  our  own  solar  system  being 

211 


THE   SUN'S   DESTINATION 

thus  illumined,  the  restless  activity  of  the  human 
intellect  was  stimulated  to  search  beyond  for  new 
problems  and  new  mysteries.  Even  more  fas- 
cinating than  the  movements  of  our  sun  and 
planets  are  all  those  questions  that  relate  to  the 
clustered  stellar  congeries  hanging  suspended 
within  the  deep  vault  of  night.  Does  the  same 
law  of  gravitation  cast  its  magic  spell  over  that 
hazy  cloud  of  Pleiades,  binding  them,  like  our- 
selves, with  bonds  indissoluble  ?  Who  shall  an- 
swer, yes  or  no  ?  We  can  only  say  that  astrono- 
mers have  as  yet  but  stepped  upon  the  threshold 
of  the  universe,  and  fixed  the  telescope's  great 
eye  upon  that  which  is  within. 

Let  us  then  begin  by  reminding  the  reader 
what  is  meant  by  the  Newtonian  law  of  gravita- 
tion. It  appears  all  things  possess  the  remark- 
able property  of  attracting  or  pulling  each  other. 
Newton  declared  that  all  substances,  solid,  liq- 
uid, or  even  gaseous — from  the  massive  cliff  of 
rock  down  to  the  invisible  air — all  matter  can 
no  more  help  pulling  than  it  can  help  existing. 
His  law  further  formulates  certain  conditions 
governing  the  manner  in  which  this  gravitational 


212 


THE  SUN'S  DESTINATION 

attraction  is  exerted ;  but  these  are  mere  matters 
of  detail ;  interest  centres  about  the  mysterious 
fact  of  attraction  itself.  How  can  one  thing  pull 
another  with  no  connecting  link  through  which 
the  pull  can  act  ?  Just  here  we  touch  the  point 
that  has  never  yet  been  explained.  Nature  with- 
holds from  science  her  ultimate  secrets.  They 
that  have  pondered  longest,  that  have  descended 
farthest  of  all  men  into  the  clear  well  of  knowl- 
edge, have  done  so  but  to  sound  the  depths  be- 
yond, never  touching  bottom. 

This  inability  of  ours,  to  give  a  good  physical 
explanation  of  gravitation,  has  led  certain  makers 
of  paradoxes  to  doubt  or  even  deny  that  there  is 
any  such  thing.  But,  fortunately,  we  have  a  sim- 
ple laboratory  experiment  that  helps  us.  Un- 
explained it  may  ever  remain,  but  that  there 
can  be  attraction  between  physical  objects  con- 
nected by  no  visible  link  is  proved  by  the  be- 
havior of  an  ordinary  magnet.  Place  a  small 
piece  of  steel  or  iron  near  a  magnetized  bar,  and 
it  will  at  once  be  so  strongly  attracted  that  it 
will  actually  fly  to  the  magnet.  Anyone  who 
has  seen  this  simple  experiment  can  never  again 

213 


THE  SUN'S  DESTINATION 

deny  the  possibility,  at  least,  of  the  law  of  attrac- 
tion as  stated  by  Newton.  Its  possibility  once 
admitted,  the  fact  that  it  can  predict  the  motions 
of  all  the  planets,  even  down  to  their  minutest 
details,  transforms  the  possibility  of  its  truth  into 
a  certainty  as  strong  as  any  human  certainty  can 
ever  be. 

But  this  demonstration  of  Newton's  law  is 
limited  strictly  to  the  solar  system  itself.  We 
may,  indeed,  reason  by  analogy,  and  take  for 
granted  that  a  law  which  holds  within  our  imme- 
diate neighborhood  is  extremely  likely  to  be  true 
also  of  the  entire  visible  universe.  But  men  of 
science  are  loath  to  reason  thus ;  and  hence  the 
fascination  of  researches  in  cosmic  astronomy. 
Analogy  points  out  the  path.  The  astronomer 
is  not  slow  to  follow ;  but  he  seeks  ever  to 
establish  upon  incontrovertible  evidence  those 
truths  which  at  first  only  his  daring  imagination 
had  led  him  to  half  suspect. 

If  we  are  to  extend  the  law  of  gravitation  to 
the  utmost,  we  must  be  careful  to  consider  the 
law  itself  in  its  most  complete  form.  A  heavenly 
body  like  the  sun  is  often  said  to  govern  the 

214 


THE  SUN'S  DESTINATION 

motions  of  its  family  of  planets  ;  but  such  a  state- 
ment is  not  strictly  accurate.  The  governing 
body  is  no  despot ;  'tis  an  abject  slave  of  law  and 
order,  as  much  as  the  tiniest  of  attendant  planets. 
The  action  of  gravitation  is  mutual,  and  no 
cosmic  body  can  attract  another  without  being 
itself  in  turn  subject  to  that  other's  gravitational 
action. 

If  there  were  in  our  solar  system  but  two  bodies, 
sun  and  planet,  we  should  find  each  one  pursu- 
ing a  path  in  space  under  the  influence  of  the 
other's  attraction.  These  two  paths  or  orbits 
would  be  oval,  and  if  the  sun  and  planet  were 
equally  massive,  the  orbits  would  be  exactly 
alike,  both  in  shape  and  size.  But  if  the  sun 
were  far  larger  than  the  planet,  the  orbits  would 
still  be  similar  in  form,  but  the  one  traversed  by 
the  larger  body  would  be  small.  For  it  is  not 
reasonable  to  expect  a  little  planet  to  keep  the 
big  sun  moving  with  a  velocity  as  great  as  that 
derived  by  itself  from  the  attraction  of  the 
larger  orb. 

Whenever  the  preponderance  of  the  larger 
body  is  extremely  great,  its  orbit  will  be  corre- 

215 


THE  SUN'S  DESTINATION 

spondingly  insignificant  in  size.  This  is  in  fact 
the  case  with  our  own  sun.  So  massive  is  it  in 
comparison  with  the  planets  that  the  orbit  is  too 
small  to  reveal  its  actual  existence  without  the 
aid  of  our  most  refined  instruments.  The  path 
traced  out  by  the  sun's  centre  would  not  fill  a 
space  as  large  as  the  sun's  own  bulk.  Neverthe- 
less, true  orbital  motion  is  there. 

So  we  may  conclude  that  as  a  necessary  con- 
sequence of  the  law  of  gravitation  every  object 
within  the  solar  system  is  in  motion.  To  say 
that  planets  revolve  about  the  sun  is  to  neglect 
as  unimportant  the  small  orbit  of  the  sun  itself. 
This  may  be  sufficiently  accurate  for  ordinary 
purposes ;  but  it  is  unquestionably  necessary  to 
neglect  no  factor,  however  small,  if  we  propose 
to  extend  our  reasoning  to  a  consideration  of  the 
stellar  universe.  For  we  shall  then  have  to  deal 
with  systems  in  which  the  planets  are  of  a  size 
comparable  with  the  sun ;  and  in  such  systems 
all  the  orbits  will  also  be  of  comparatively  equal 
importance. 

Mathematical  analysis  has  derived  another  fact 
from  discussion  of  the  law  of  gravitation  which, 

216 


THE  SUN'S  DESTINATION 

perhaps,  transcends  in  simple  grandeur  every- 
thing we  have  as  yet  mentioned.  It  matters  not 
how  great  may  be  the  number  of  massive  orbs 
threading  their  countless  interlacing  curved  paths 
in  space,  there  yet  must  be  in  every  cosmic  sys- 
tem one  single  point  immovable.  This  point  is 
called  the  Centre  of  Gravity.  If  it  should  so 
happen  that  in  the  beginning  of  things,  some 
particle  of  matter  were  situated  at  this  centre, 
then  would  that  atom  ever  remain  unmoved  and 
imperturbable  throughout  all  the  successive  vicis- 
situdes of  cosmic  evolution.  It  is  doubtful 
whether  the  mind  of  man  can  form  a  conception 
of  anything  grander  than  such  an  immovable 
atom  within  the  mysterious  intricacies  of  cosmic 
motion. 

But  in  general,  we  cannot  suppose  that  the 
centres  of  gravity  in  the  various  stellar  systems 
are  really  occupied  by  actual  physical  bodies. 
The  centre  may  be  a  mere  mathematical  point  in 
space,  situated  among  the  several  bodies  compos- 
ing the  system,  but,  nevertheless,  endowed,  in  a 
certain  sense,  with  the  same  remarkable  property 

of  relative  immobility. 

217 


THE   SUN'S   DESTINATION 

Having  thus  defined  the  centre  of  gravity  in 
its  relation  to  the  constituent  parts  of  any  cosmic 
system,  we  can  pass  easily  to  its  characteristic 
properties  in  connection  with  the  inter-relation 
of  stellar  systems  with  one  another.  It  can  be 
proved  mathematically  that  our  solar  system  will 
pull  upon  distant  stars  just  as  though  the  sun 
and  all  the  planets  were  concentrated  into  one 
vast  sphere  having  its  centre  in  the  centre  of 
gravity  of  the  whole.  It  is  this  property  of  the 
centre  of  gravity  which  makes  it  pre-eminently 
important  in  cosmic  researches.  For,  while  we 
know  that  centre  to  be  at  rest  relatively  to  all 
the  planets  in  the  system,  it  may,  nevertheless, 
in  its  quality  as  a  sort  of  concentrated  essence  of 
them  all,  be  moving  swiftly  through  space  under 
the  pull  of  distant  stars.  In  that  case,  the  at- 
tendant bodies  will  go  with  it — but  they  will 
pursue  their  evolutions  within  the  system,  all  un- 
conscious that  the  centre  of  gravity  is  carrying 
them  on  a  far  wider  circuit. 

What  is  the  nature  of  that  circuit?  This 
question  has  been  for  many  years  the  subject  of 
earnest  study  by  the  clearest  minds  among  as- 

218 


THE   SUN'S   DESTINATION 

tronomers.  The  greatest  difficulty  in  the  way  is 
the  comparatively  brief  period  during  which  men 
have  been  able  to  make  astronomical  observa- 
tions of  precision.  Space  and  time  are  two  con- 
ceptions that  transcend  the  powers  of  definition 
possessed  by  any  man.  But  we  can  at  least 
form  a  notion  of  how  vast  is  the  extent  of  time, 
if  we  remember  that  the  period  covered  by  man's 
written  records  is  registered  but  as  a  single  mo- 
ment upon  the  great  revolving  dial  of  heaven's 
dome.  One  hundred  and  fifty  years  have 
elapsed  since  James  Bradley  built  the  founda- 
tions of  modern  sidereal  astronomy  upon  his  mas- 
terly series  of  observations  at  the  Royal  Observa- 
tory of  Greenwich,  in  England.  Yet  so  slowly 
do  the  movements  of  the  stars  unroll  themselves 
upon  the  firmament,  that  even  to  this  day  no 
one  of  them  has  been  seen  by  men  to  trace  out 
more  than  an  infinitesimal  fraction  of  its  destined 
path  through  the  voids  of  space. 

Travellers  upon  a  railroad  cannot  tell  at  any 
given  moment  whether  they  are  moving  in  a 
straight  line,  or  whether  the  train  is  turning 
upon  some  curve  of  huge  size.  The  St.  Goth- 

219 


THE  SUN'S  DESTINATION 

ard  railway  has  several  so-called  "  corkscrew " 
tunnels,  within  which  the  rails  make  a  complete 
turn  in  a  spiral,  the  train  finally  emerging  from 
the  tunnel  at  a  point  almost  vertically  over  the 
entrance.  In  this  way  the  train  is  lifted  to  a 
higher  level.  Passengers  are  wont  to  amuse 
themselves  while  in  these  tunnels  by  watching 
the  needle  of  an  ordinary  pocket-compass.  This 
needle,  of  course,  always  points  to  the  north  ; 
and  as  the  train  turns  upon  its  curve,  the  needle 
will  make  a  complete  revolution.  But  the  pas- 
senger could  not  know  without  the  compass 
that  the  train  was  not  moving  in  a  perfectly 
straight  line.  Just  so  we  passengers  on  the 
earth  are  unaware  of  the  kind  of  path  we  are 
traversing,  until,  like  the  compass,  the  astron- 
omer's instruments  shall  reveal  to  us  the 
truth. 

But  as  we  have  seen,  astronomical  observations 
of  precision  have  not  as  yet  extended  through  a 
period  of  time  corresponding  to  the  few  minutes 
during  which  the  St.  Gothard  traveller  watches 
the  compass.  We  are  still  in  the  dark,  and  do 
not  know  as  yet  whether  mankind  shall  last  long 

220 


THE   SUN'S   DESTINATION 

enough  upon  the  earth  to  see  the  compass  needle 
make  its  revolution.  We  are  compelled  to  be- 
lieve that  the  motion  in  space  of  our  sun  is  pro- 
gressing upon  a  curved  path ;  but  so  far  as  pre- 
cise observations  allow  us  to  speak,  we  can  but 
say  that  we  have  as  yet  moved  through  an  infini- 
tesimal element  only  of  that  mighty  curve.  How- 
ever, we  know  the  point  upon  the  sky  toward 
which  this  tiny  element  of  our  path  is  directed, 
and  we  have  an  approximate  knowledge  of  the 
speed  at  which  we  move. 

More  than  a  century  ago  Sir  William  Herschel 
was  able  to  fix  roughly  what  we  call  the  apex  of 
the  sun's  way  in  space,  or  the  point  among  the 
stars  toward  which  that  way  is  for  the  moment 
directed.  We  say  for  the  moment,  but  we  mean 
that  moment  of  which  Bradley  saw  the  beginning 
in  1750,  and  upon  whose  end  no  man  of  those 
now  living  shall  ever  look.  Herschel  found  that 
a  comparison  of  old  stellar  observations  seemed 
to  indicate  that  the  stars  in  a  certain  part  of  the 
sky  were  opening  out,  as  it  were,  and  that  the 
constellations  in  the  opposite  part  of  the  heavens 
seemed  to  be  drawing  in,  or  becoming  smaller. 

221 


THE  SUN'S   DESTINATION 

There  can  be  but  one  reaspnable  explanation  of 
this.  We  must  be  moving  toward  that  part  of 
the  sky  where  the  stars  are  separating.  Just  so  a 
man  watching  a  regiment  of  soldiers  approaching, 
will  see  at  first  only  a  confused  body  of  men ; 
but  as  they  come  nearer,  the  individual  soldiers 
will  seem  to  separate,  until  at  length  each  one  is 
seen  distinct  from  all  the  others. 

Herschel  fixed  the  position  of  the  apex  at  a 
point  in  the  constellation  Hercules.  The  most 
recent  investigations  of  Newcomb  and  others 
have,  on  the  whole,  verified  Herschel's  conclu- 
sions. With  the  intuitive  power  of  rare  genius, 
Herschel  had  been  able  to  sift  truth  out  of  error. 
The  observational  data  at  his  disposal  would  now 
be  called  rude,  but  they  disclosed  to  the  scrutiny 
of  his  acute  understanding  the  germ  of  truth  that 
was  in  them.  Later  investigators  have  increased 
the  precision  of  our  knowledge,  until  we  can  now 
say  that  the  present  direction  of  the  solar  motion 
is  known  within  very  narrow  limits.  A  tiny  circle 
might  be  drawn  on  the  sky,  to  which  an  astrono- 
mer might  point  his  hand  and  say  :  "Yonder  little 
circle  contains  the  goal  toward  which  the  sun  and 


222 


THE   SUN'S   DESTINATION 

planets  are  hastening  to-day."  Even  the  speed 
of  this  motion  has  been  subjected  to  measure- 
ment, and  found  to  be  about  ten  miles  per 
second. 

The  objective  point  and  the  rate  of  motion 
thus  stated,  exact  science  holds  her  peace.  Here 
genuine  knowledge  stops  ;  and  we  can  proceed 
further  only  by  the  aid  of  that  imagination  which 
men  of  science  need  to  curb  at  every  moment. 
But  let  no  one  think  that  the  sun  will  ever 
reach  the  so-called  apex.  To  do  so  would  mean 
cosmic  motion  upon  a  straight  line,  while  every 
consideration  of  celestial  mechanics  points  to  mo- 
tion upon  a  curve.  When  shall  we  turn  suffi- 
ciently upon  that  curve  to  detect  its  bending  ? 
'Tis  a  problem  we  must  leave  as  a  rich  heritage 
to  later  generations  that  are  to  follow  us.  The 
visionary  theorist's  notion  of  a  great  central  sun, 
controlling  our  own  sun's  way  in  space,  must  be 
dismissed  as  far  too  daring.  But  for  such  a  cen- 
tral sun  we  may  substitute  a  central  centre  of 
gravity  belonging  to  a  great  system  of  which  our 
sun  is  but  an  insignificant  member.  Then  we 
reach  a  conception  that  has  lost  nothing  in  the 

223 


THE  SUN'S   DESTINATION 

grandeur  of  its  simplicity,  and  is  yet  in  accord 
with  the  probabilities  of  sober  mechanical  science. 
We  cease  to  be  a  lonely  world,  and  stretch  out 
the  bonds  of  a  common  relationship  to  yonder 
stars  within  the  firmament. 


UNIVERSITY 

OF 


224 


INDEX 

PAGE 

Airy,  Astronomer  Royal l 

Allis,  photographs  comet IO1 

Andromeda  nebula 28 

temporary  star 28,  29,  45 

Apex,  of  solar  motion,  explained 221 

Aquila,  constellation,  temporary  star  in 40 

Arctic  regions,  position  of  pole  in 194 

Argo,  constellation,  variable  star  in r 205 

Association,   international  geodetic 139 

Asteroids,  first  discovery  by  Piazzi 59>  Io6 

discovery  by  photography 64 

group  of 63 

photography  of,  invented  by  Wolf IO4 

Astronomer,    royal J 

working,  description  of I52 

ASTRONOMER'S  POLE,  THE 184 

Astronomy,  journalistic 176 

practical  uses  of 112 

Atmospheric  refraction,  explained 193 

Axis,  of  figure  of  the  earth 136 

of  rotation  of  the  earth 136 

polar,  of  telescope 173 

Barnard,  discovers  satellite  of  Jupiter 51 

Bessel,  measures  Pleiades 15 

Bond,  discovers  crape  ring  of  Saturn 144 

Bradley,  observes  at  Greenwich 219 

Brahe,  Tycho,  his  temporary  star 40 

Bruce,  endows  polar  photography 197 

225 


INDEX 

PAGE 

Campbell,  observes  Pole-star 18 

Cape  of  Good  Hope,  observatory,  photography  at 101 

telescope 1 7°>  1 74 

Capriccio,  Galileo's 55 

Cassini,  shows   Saturn's  rings  to  be  double H4 

Cassiopeia,  temporary  star  in 4° 

Celestial  pole 184 

Central  sun  theory 223 

Centre  of  gravity 217 

Chart-room,  on  ship-board 5 

Chronometer,  invention  of & 

Circle,  meridian,  explained 189 

Clerk-Maxwell,  discusses  Saturn's  rings 146 

Clock,  affected  by  temperature 117 

affected  by  barometric  pressure 117 

astronomical 115 

astronomical,  how  mounted 1 16 

astronomical,  its  dial 1 16 

error  of,  determined  with  transit 1 18 

jeweller's   regulator 114 

of  telescope 175 

Clusters  of  stars,  photography  of 98 

Columbia  University  Observatory,  latitude  observations 139 

polar   photography 196 

Common,  his  reflecting  telescope 32 

Confusion  of  dates,  in  Pacific  Ocean 125 

Congress  of  Astronomers,  Paris,  1887 102 

Constellations 162 

Control,    "  mouse,"  for  photography 88 

Copernican  theory  of  universe 53,  56 

demonstration 94 

Corkscrew  tunnels 220 

Crape  ring  of  Saturn 144 

Cumulative  effect,  in  photography ,    84 

Date,  confusion  of,  in  Pacific  Ocean 125 

Date-line,   international,   explained 126 

Development  of  photograph  81 

226 


INDEX 

PAGE 

Dial,  of  astronomical  clock 116 

"Dialogue  "  of  Galileo 53 

Differences  of  time,  explained 121 

Directions,  telescopic  measurement  of 21 

Directory  of  the  heavens 103 

Distance,  of  light-source  in  photography 83 

of  stars 94,  106,  1 58 

of  vSun 67,  97,  106 

Donner,  polar  photography 195 

Double  telescopes,  for  photography 86 

Earth,  motions  of  its  pole 131 

rotation  of 136,  162,  171,  184 

shape  of 135 

Eclipses,  photography  of 109 

Elkin,  measures  Pleiades 15 

Equatorial  telescope,  explained 170 

Eros,  discovered  by  Witt , .   66,  105 

its  importance 67 

Error  of  clock,  determined  by  transit 1 18 

Exposure,  length  of,  in  photography 84 

Feldhausen,  Herschel's  observatory  near  Capetown 204 

Fiji  Islands,  their  date 126 

Fixed  polar  telescope 197 

"  Following  "  the  stars 88,  173 

Four-day  cycle  of  pole-star 24 

France,  outside  time-zone  system 129 

Fundamental  longitude  meridian 124 

GALILEO 47 

and  the  Church 48 

discoveries  of 49 

observes  Saturn 141 

Galle,  discovers  Neptune 61 

Gauss,  computes  first  asteroid  orbit 60 

Gautier,  Paris,  constructs  big  telescope 179 

Geodetic  Association,  international , 139 

227 


INDEX 

PAGE 

Geography,  maps,  astronomical  side  of 112 

Geology,  polar  motion  in 13' 

Gill,  photographs  comet 100 

Gilliss,  at  Naval  Observatory,  Washington 169 

Goldsborough,  at  Naval  Observatory,  Washington 169 

Grande  Lunette,  Paris,  1900 17°>  180 

Gravitation 13 

in  Pleiades 14,  212 

law  of,  Newton's 212 

Gravity,  centre  of 217 

Greenwich,  origin  of  longitudes 7»  I24 

time 7 

Groombridge,  English  astronomer I 

Harrison,  inventor  of  chronometer 8 

Head,  of  heliometer 156 

Heidelberg,  photography  at 104 

HELIOMETER 152 

head  of 156 

how  used 157 

principle  of 154 

scales  of 158 

semi-lenses  of 155 

Helsingfors  observatory,  polar  photography  at 195 

Henry,  measures  Pleiades II,  17 

Hercules,  constellation,  solar  motion  toward 222 

Herschel,  discovers  apex  of  solar  motion 221 

discovers  Uranus 59,  141 

John,  the  moon  hoax 200 

Hipparchus,  discovers  precession 186 

early  star  catalogue 21,  39 

invents  star  magnitudes „ 91 

Huygens,  announces  rings  of  Saturn 142 

his  logogriph 143 

Ice-cap,  of  Earth 131 

Index  Librorum  Prohibitorum 53 

International,  date-line,  explained 126 

geodetic  association 139 

228 


INDEX 

PAGE 

Inter-stellar  motion,  in  clusters 98 

in  Pleiades 14 

Islands  of  Pacific,  their  longitude  and  time 125 

Japan,  latitude  station  in 139 

Jewellers'  correct  time 1 2 1 

Journalistic  astronomy 176 

Jupiter's  satellites,  discovered  by  Galileo 50 

discovered  by  Barnard 51 

Keeler,  observes  Saturn's  rings 140,  147,  150 

photographs  nebulas 32 

"  Keyhole  "    nebula 205 

Lambert,  determines  longitude  of  Washington 168 

Laplace,  discusses  Saturn's  rings 146 

nebular  hypothesis 33 

stability  of  solar  system 210 

Latitude,  changes  of 133,  138 

definition  of 134 

determining  the 6; 

Leverrier,  predicts  discovery  of  Neptune 61,  142 

Lick  Observatory,  Keeler's  observations 140 

Light,  undulatory  theory  of 19,  148 

Light-waves,  measuring  length  of 20,  149 

Logogriph,  by  Huygens 143 

Long- exposure  photography 85 

Longitude,  counted  East  and  West 125 

determining 6 

determining  by  occultations 167 

effect  on  time  differences 123 

explained 123 

of  Washington,  first  determined 168 

Maclear,  observes  Eta  Argus 205 

Magnitudes,  stellar 91 

Manila,    its  time 127 

Maps,  astronomical  side  of H2 

229 


INDEX 

PAGE 

Meridian  circle,  explained 189 

Milky- way,  poor  in  nebulae 33 

Minor  Planets,  see  Asteroids. 

MOON,  HOAX 199 

motion  among  stars 163 

mountains  discovered  by  Galileo 49 

size  of,  measured 166 

Motion  of  moon 163 

MOTIONS  of  the  EARTH'S  Pole 131 

MOUNTING  GREAT  TELESCOPES 170 

Naked-eye  nebulae 28 

Naples,  Royal  Observatory,  latitude  observations 139 

Naval  Observatory,  Washington,  noon  signal 120 

NAVIGATION i 

before  chronometers 3 

use  of  astronomy  in 113 

NEBULA 27 

Nebula,  in  Andromeda 28 

in  Orion 30 

"  keyhole  " 205 

Nebular,  hypothesis 33 

structure  in  Pleiades 17 

Nebulous  stars 31 

Negative,  and  positive,  in  photography 82 

Neptune,  discovery  predicted  by  Leverrier 61,  142 

discovery  by  Galle 61 

Newcomb,  fixes  apex  of  solar  motion 222 

Newton,  law  of  gravitation 212 

longitude  commission 8 

New  York,  its  telegraphic  time  system 120 

Noon  Signal,  Washington .  1 20 

Number,  of  nebulae 31,  33 

of  temporary  stars , . . .  38 

Nutation,  explained 188 

Occultations 161 

explained 165 

230 


INDEX 

PACK 

Occultations,  use  of 166,  167 

Orion  nebula 30 

Pacific  islands,  their  longitude  and  time 125 

Parallax,  solar 67,  106 

stellar 94,  106 

measured  with  heliometer 158 

Paris,  congress  of  astronomers,  1887 102 

exposition  of  1900 1 76 

Periodic  motion  of  earth's  pole 133 

Perseus,  constellation,  temporary  star  in 46 

Philippine  Islands,  their  time 127 

Photography,  asteroid,  invented  by  Wolf 104 

congress  of  astronomical 102 

cumulative  effect  of  light 84 

distance  of  light-source 83 

double  telescopes  for 86 

general  star  catalogue 102 

IN  ASTRONOMY 8l 

in  discovery  of  asteroids 64,  104 

in  solar  physics 109 

in  spectroscopy 108 

length  of  exposure 84 

measuring  machine,  Rutherfurd 93 

motion  of  telescope  for. 87 

"mouse  "  control  of  telescope 88 

of  eclipses 109 

of  inter-stellar  motion 99 

Paris  congress,  1877 102 

polar 191 

Rutherfurd  pioneer  in 90 

star-clusters 98 

star-distances  measured  by 94 

summarized no 

wholesale  methods  in 103 

Piazzi,  discovers  first  asteroid 59«  IQ6 

Pitkin,  report  to  House  of  Representatives 168 

Planetary  nebulas 31 

231 


INDEX 

PAGE 

PLANET  OF  1898 58 

Planetoids,  see  Asteroids. 

Planets  known  to  ancients 58 

PLEIADES 10 

gravitation  among 212 

motion  among 14,  16,  98 

nebular  structure 17 

number  visible 1 1 

Polar  axis,  of  telescope 1 73 

Polar  photography 191 

at  Helsingfors 195 

Pole,  celestial 184 

of  the  earth,  motions  of 131 

THE  ASTRONOMER'S 184 

POLE-STAR , 18 

as  a  binary 25 

as  a  triple 18,  26 

change  of 187 

its  four-day  cycle 24 

motion  toward  us 24 

Positive,  and  negative,  in  photography 82 

Potsdam,  observatory,  photographic  star-catalogue 103 

Practical  uses  of  astronomy 112 

Precession,  explained 186 

Prize,  for  invention  of  chronometer 8 

Ptolemaic  theory  of  universe 56 

Ptolemy,  writes  concerning  Hipparchus 39 

Railroad  time,  explained 127 

Refraction,  atmospheric,  explained 193 

"  Regulator,"  the  jeweller's  clock 114 

Ring-nebulae 31 

Rings,  of  Saturn,  see  Saturn's  rings. 

Roberts,  Andromeda  nebula 28 

Rotation,  of  Earth 136,  162,  171,  184 

of  Saturn 150 

Royal  Astronomer,  his  duties. 2 

Royal  Observatory,  Greenwich 124 

232 


INDEX 

PAGE 

Greenwich,  Bradley's  observations 219 

Naples,  latitude  observations 1 39 

Rutherfurd,  cluster  photography 99 

invents  photographic  apparatus 93 

pioneer  in  photography 90 

stellar  parallax 94 

Sagredus,  character  in  Galileo's  Dialogue 55 

Salusbury,  Galileo's  translator 50,  54 

Salviati,  character  in  Galileo's  Dialogue 55 

Samoa,  its  date 126 

SATURN'S  RINGS 140 

analogy  to  planetoids 147 

announced  by  Huygens 142 

observed  with  spectroscope 147 

shown  to  be  double  by  Cassini. 144 

structure  and  stability 145 

Scales,  of  heliometer 158 

Scorpio,  constellation,  temporary  star  in 39 

Semi-lenses  of  heliometer 155 

Sextant,  how  used 4 

Sicily,  latitude  station  in 139 

Sidereus  Nuncius,  published  by  Galileo 52 

Simplicio,  character  in  Galileo's  Dialogue 55 

Sirius,  brightest  star 205 

Size  of  Moon,  measured 166 

Socie'td  de  I'Optique 177 

Solar  parallax,  see  Sun's  distance. 

physics,  by  photography 109 

system,  stability  of 210 

Spectroscope,  its  use  explained 147 

used  on  pole-star 19 

to  observe  Saturn's  rings. 147 

Spiral  nebulae , 31 

Stability,  of  Saturn's  rings 145 

of  Solar  System.  . , 210 

Standards,  time,  of  the  world 1 1 1 

table  of 130 

233 


INDEX 

PAGE 

•'  Standard  "  time,  explained 127 

Star-catalogue,  general  photographic 102 

Star-clusters,  photography  of 98 

Star-distances 94,  106 

measured  with  heliometer 158 

Rutherfurd 94 

Star  magnitudes 91 

Star-motion,  toward  us 21 

Star-tables,  astronomical 118 

Stars,  variable 42 

St.  Gothard  railway,  tunnels 220 

Sun,  newspaper,  the  moon  hoax 201 

SUN-DIAL,  How  TO  MAKE  A 69 

SUN'S,  DESTINATION 210 

distance,  compared  with  star  distance 97 

measured  with  Eros 67,  106 

motion,  apex  of 221 

Sun-spots,  discovered  by  Galileo 49 

Systema  Saturnium^  Huygens 143 

Telescope,  clock 175 

at  Paris  Exposition 1 76,  180 

double,  for  photography 86 

equatorial,  explained 170 

first  used  by  Galileo 49 

motion  of 87 

mounting  great 1 70 

unmoving,  for  polar  photography 197 

TEMPORARY  STARS 37 

in  Andromeda  nebula 28,  29,  45 

in  Aquila 40 

in  Cassiopeia 40 

in  Perseus 46 

in  Scorpio 39 

their  number 38 

theory  of 42 

Time,  correct,  determined  astronomically 113 

differences  between  different  places 121 

234 


INDEX 

PAGE 

TIME  STANDARDS  OF  THE  WORLD 1 1 1 

standards  of  the  World,  table  of 130 

system,  in  New  York 120 

zones,  explained. 128 

Trails,  photographic 191 

Transit,  for  determining  clock  error 1 18 

Tycho  Brahe,  his  temporary  star 40 

Ulugh  Beg,  early  star  catalogue 21 

Undulatory  theory,  of  light 19,  148 

Universe,  theories  of 34,  53,  56 

Uranus,  discovered  by  Herschel 59,  142 

Use  of  occultations 166,  167 

Uses  of  astronomy,  practical 1 12 

Variable  stars 42 

in  Argo 205 

Vega,  future  pole-star 187 

Visibility  of  stars,  in  day-time 191 

Vision,  phenomenon  of 20,  149 

Washington,  its  longitude  first  determined 168 

Waves,  explained 148 

of  light 20,  148 

Wilkes,  at  Naval  Observatory,  Washington 169 

Wilkins,  imaginary  voyage  of 208 

Witt,  discovers  Eros 66,  105 

Wolf,  M.,  invents  asteroid  photography 104 

measures  Pleiades 1 1 

World's  time  standards,  table  of 130 

Yale  College,  Pleiades  measured  at 15 

Zones,  time,  explained 128 


335 


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101613 


